Add a PR Checklist for Contributions (#7921)

# Description

Adding a contribution PR checklist to ensure important areas like,
design, samples, and doc updates are covered.

## Type of change

<!--

Please select **one** of the following options that describes your
change and delete the others. Clearly identifying the type of change you
are making will help us review your PR faster, and is used in authoring
release notes.

If you are making a bug fix or functionality change to Radius and do not
have an associated issue link please create one now.

-->

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).

<!--

Please update the following to link the associated issue. This is
required for some kinds of changes (see above).

-->

---------

Signed-off-by: karishma-chawla <74574173+karishma-chawla@users.noreply.github.com>
Co-authored-by: karishma-chawla <74574173+karishma-chawla@users.noreply.github.com>

.github/pull_request_template.md

@@ -23,3 +23,13 @@ Please update the following to link the associated issue. This is required for s
 -->
 
 Fixes: #issue_number
+
+## Contributor checklist
+Please verify that the PR meets the following requirements, where applicable:
+
+- [ ] An overview of proposed schema changes is included in a linked GitHub issue.
+- [ ] A design document PR is created in the [design-notes repository](https://github.com/radius-project/design-notes/), if new APIs are being introduced.
+- [ ] If applicable, design document has been reviewed and approved by Radius maintainers/approvers.
+- [ ] A PR for the [samples repository](https://github.com/radius-project/samples) is created, if existing samples are affected by the changes in this PR.
+- [ ] A PR for the [documentation repository](https://github.com/radius-project/docs) is created, if the changes in this PR affect the documentation or any user facing updates are made.
+- [ ] A PR for the [recipes repository](https://github.com/radius-project/recipes) is created, if existing recipes are affected by the changes in this PR.

docs/contributing/contributing-pull-requests/README.md

@@ -23,6 +23,7 @@ A pull request will need to pass the following checkpoints to be accepted:
 
 - Initial review: a maintainer will review your summary and make sure an appropriate issue is linked
 - Testing: automated tests will run against your changes
+- Contributor PR checklist: the PR meets the [checklist requirements](https://github.com/radius-project/radius/blob/main/.github/pull_request_template.md#contributor-checklist).
 - Code review: you will get feedback from a maintainer or other contributors in the form of comments
 
 We expect that contributors have run basic validations (`make build test lint`) before sending a pull request. See [building the repo](../contributing-code/contributing-code-building/) for more information. If you get stuck during this step feel free to open the pull request anyway and ask for help in our [forum](https://discordapp.com/channels/1113519723347456110/1115302284356767814).