Update contributing guide with new label policy (paritytech#6333)

* mention C and M labels in contributing guide

* update PR template with more specific instructions

* update PR template with updated label rules and contributing guide link

* update contibuting guide

docs/CONTRIBUTING.adoc

@@ -19,17 +19,30 @@ There are a few basic ground-rules for contributors (including the maintainer(s)
 
 == Merge Process
 
-Merging pull requests once CI is successful:
+*In General*
 
-. A PR needs to be reviewed and approved by project maintainers unless:
-	- it does not alter any logic (e.g. comments, dependencies, docs), then it may be tagged https://github.com/paritytech/substrate/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen+label%3AA2-insubstantial[`insubstantial`] and merged by its author once CI is complete.
-  - it is an urgent fix with no large change to logic, then it may be merged after a non-author contributor has approved the review once CI is complete.
+A PR needs to be reviewed and approved by project maintainers unless:
 
+- it does not alter any logic (e.g. comments, dependencies, docs), then it may be tagged https://github.com/paritytech/substrate/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen+label%3AA2-insubstantial[`insubstantial`] and merged by its author once CI is complete.
+- it is an urgent fix with no large change to logic, then it may be merged after a non-author contributor has approved the review once CI is complete.
+
+*Labels TLDR:*
+
+- `A-*` Pull request status. ONE REQUIRED.
+- `B-*` Changelog and/or Runtime-upgrade post composition markers. ONE REQUIRED. (used by automation)
+- `C-*` Release notes release-priority markers. EXACTLY ONE REQUIRED. (used by automation)
+- `D-*` More general tags on the PR denoting various implications and requirements.
+
+*Process:*
+
+. Please tag each PR with exactly one `A`, `B` and `C` label at the minimum.
 . Once a PR is ready for review please add the https://github.com/paritytech/substrate/pulls?q=is%3Apr+is%3Aopen+label%3AA0-pleasereview[`A0-pleasereview`] label. Generally PRs should sit with this label for 48 hours in order to garner feedback. It may be merged before if all relevant parties had a look at it.
 . If the first review is not an approval, swap `A0-pleasereview` to any label `[A3, A7]` to indicate that the PR has received some feedback, but needs further work. For example. https://github.com/paritytech/substrate/labels/A3-inprogress[`A3-inprogress`] is a general indicator that the PR is work in progress and https://github.com/paritytech/substrate/labels/A4-gotissues[`A4-gotissues`] means that it has significant problems that need fixing. Once the work is done, change the label back to `A0-pleasereview`. You might end up swapping a few times back and forth to climb up the A label group. Once a PR is https://github.com/paritytech/substrate/labels/A8-mergeoncegreen[`A8-mergeoncegreen`], it is ready to merge. 
 . PRs must be tagged with respect to _release notes_ with https://github.com/paritytech/substrate/labels/B0-silent[`B0-silent`] and `B1-..`. The former indicates that no changes should be mentioned in any release notes. The latter indicates that the changes should be reported in the corresponding release note
-. PRs that break the external API must be tagged with https://github.com/paritytech/substrate/labels/B2-breaksapi[`B2-breaksapi`], when it changes the FRAME or consensus of running system with https://github.com/paritytech/substrate/labels/B3-breaksconsensus[`B3-breaksconsensus`]
-. No PR should be merged until all reviews' comments are addressed.
+. PRs that break the external API must be tagged with https://github.com/paritytech/substrate/labels/B2-breaksapi[`B2-breaksapi`], when it changes the FRAME or consensus of running system with https://github.com/paritytech/substrate/labels/B3-breaksconsensus[`B3-breaksconsensus`].
+. PRs should be labeled with their release importance via the `C1-C9`.
+. PRs should be categorized into projects.
+. No PR should be merged until all reviews' comments are addressed and CI is successful.
 
 *Reviewing pull requests*:
 
@@ -49,7 +62,7 @@ When reviewing a pull request, the end-goal is to suggest useful changes to the
 
 === Updating Polkadot as well
 
-**All pull requests will be checked agains either Polkadot master, or your provided Polkadot companion PR**. That is, If your PR changes the external APIs or interfaces used by Polkadot. If you tagged the PR with `breaksapi` or `breaksconsensus` this is most certainly the case, in all other cases check for it by running step 1 below.
+**All pull requests will be checked against either Polkadot master, or your provided Polkadot companion PR**. That is, If your PR changes the external APIs or interfaces used by Polkadot. If you tagged the PR with `breaksapi` or `breaksconsensus` this is most certainly the case, in all other cases check for it by running step 1 below.
 
 To create a Polkadot companion PR:
 
@@ -69,6 +82,14 @@ As there might be multiple pending PRs that might conflict with one another, a)
 
 We use https://github.com/paritytech/substrate/labels[labels] to manage PRs and issues and communicate state of a PR. Please familiarize yourself with them. Furthermore we are organizing issues in https://github.com/paritytech/substrate/milestones[milestones]. Best way to get started is to a pick a ticket from the current milestone tagged https://github.com/paritytech/substrate/issues?q=is%3Aissue+is%3Aopen+label%3AQ2-easy[`easy`] or https://github.com/paritytech/substrate/issues?q=is%3Aissue+is%3Aopen+label%3AQ3-medium[`medium`] and get going or https://github.com/paritytech/substrate/issues?q=is%3Aissue+is%3Aopen+label%3AX1-mentor[`mentor`] and get in contact with the mentor offering their support on that larger task.
 
+== Issues
+Please label issues with the following labels:
+
+. `I-*` Issue severity and type. EXACTLY ONE REQUIRED.
+. `P-*` Issue priority. AT MOST ONE ALLOWED.
+. `Q-*` Issue difficulty. AT MOST ONE ALLOWED.
+. `Z-*` More general tags on the issue, denoting context and resolution.
+
 == Releases
 
 Declaring formal releases remains the prerogative of the project maintainer(s).

docs/PULL_REQUEST_TEMPLATE.md

@@ -6,17 +6,24 @@ Before you submitting, please check that:
   - What does it do?
   - What important points reviewers should know?
   - Is there something left for follow-up PRs?
-- [ ] You labeled the PR with appropriate labels if you have permissions to do so.
+- [ ] You labeled the PR appropriately if you have permissions to do so:
+  - [ ] `A*` for PR status (**one required**)
+  - [ ] `B*` for changelog (**one required**)
+  - [ ] `C*` for release notes (**exactly one required**)
+  - [ ] `D*` for various implications/requirements
+  - [ ] Github's project assignment
 - [ ] You mentioned a related issue if this PR related to it, e.g. `Fixes #228` or `Related #1337`.
 - [ ] You asked any particular reviewers to review. If you aren't sure, start with GH suggestions.
-- [ ] Your PR adheres [the style guide](https://wiki.parity.io/Substrate-Style-Guide)
-  - In particular, mind the maximal line length.
+- [ ] Your PR adheres to [the style guide](https://wiki.parity.io/Substrate-Style-Guide)
+  - In particular, mind the maximal line length of 100 (120 in exceptional circumstances).
   - There is no commented code checked in unless necessary.
   - Any panickers have a proof or removed.
 - [ ] You bumped the runtime version if there are breaking changes in the **runtime**.
 - [ ] You updated any rustdocs which may have changed
 - [ ] Has the PR altered the external API or interfaces used by Polkadot? Do you have the corresponding Polkadot PR ready?
 
+Refer to [the contributing guide](https://github.com/paritytech/substrate/blob/master/docs/CONTRIBUTING.adoc) for details.
+
 After you've read this notice feel free to remove it.
 Thank you!