chore: add a linter for pull request titles (microsoft#3500)

<!--- Provide a summary of your changes in the title field above. For guidance on formatting, see the comment at the bottom of this template. -->

# Description

<!--- Describe your changes. -->
This change adds an action that will lint the pull request titles to ensure the follow conventional commits. It also adds in some contributor documentation for dealing with merging PRs.

## Motivation & context

<!--- What problem does this change solve? -->
<!--- Provide a link if you are addressing an open issue. -->
Too many commits are going in without the proper commit messages, this will result in erroneous change logs as well as incorrect versioning.

## Issue type checklist

<!--- What type of change are you submitting? Put an x in the box that applies: -->

- [x] **Chore**: A change that does not impact distributed packages.
- [ ] **Bug fix**: A change that fixes an issue, link to the issue above.
- [ ] **New feature**: A change that adds functionality.

**Is this a breaking change?**
- [ ] This change causes current functionality to break.

<!--- If yes, describe the impact. -->

**Adding or modifying component(s) in `@microsoft/fast-components` checklist**

<!-- Do your changes add or modify components in the @microsoft/fast-components package? Put an x in the box that applies: -->

- [ ] I have added a new component
- [ ] I have modified an existing component
- [ ] I have updated the [definition file](https://github.com/Microsoft/fast/blob/master/packages/web-components/fast-components/CONTRIBUTING.md#definition)
- [ ] I have updated the [configuration file](https://github.com/Microsoft/fast/blob/master/packages/web-components/fast-components/CONTRIBUTING.md#configuration)

## Process & policy checklist

<!--- Review the list and check the boxes that apply. -->

- [ ] I have added tests for my changes.
- [ ] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/Microsoft/fast/blob/master/CONTRIBUTING.md) documentation and followed the [standards](https://www.fast.design/docs/en/contributing/standards) for this project.

<!---
Formatting guidelines:

Accepted peer review title format:
<type>: <description>

Example titles:
    chore: add unit tests for all components
    feat: add a border radius to button
    fix: update design system to use 3px border radius

    <type> is required to be one of the following:

        - chore: A change that does not impact distributed packages.
        - fix: A change which fixes an issue.
        - feat: A that adds functionality.

    <description> is required for the CHANGELOG and speaks to what the user gets from this PR:

        - Be concise.
        - Use all lowercase characters. 
        - Use imperative, present tense (e.g. `add` not `adds`.)
        - Do not end your description with a period.
        - Avoid redundant words.

For additional information regarding working on FAST, check out our documentation site:
https://www.fast.design/docs/en/contributing/working
-->

.github/workflows/pr.yml

@@ -0,0 +1,17 @@
+name: "PR - FAST"
+on:
+  pull_request:
+    types:
+      - opened
+      - reopened
+      - edited
+      - synchronize
+
+jobs:
+  lint_title:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: amannn/action-semantic-pull-request@v1.2.0
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

CONTRIBUTING.md

@@ -66,6 +66,10 @@ Packages are located within the `packages` folder of the repository. Each packag
 
 If you'd like to contribute by fixing a bug, implementing a feature, or even correcting typos in our documentation, you'll want to submit a pull request. Before submitting a pull request, be sure to [rebase](https://www.atlassian.com/git/tutorials/merging-vs-rebasing) your branch from master. Do not use ``git merge`` or the *merge* button provided by GitHub.
 
+### Merging a pull request
+
+If you are merging a pull request, be sure to use the pull request title as the commit title. The title should follow the [conventional commit guidelines](https://www.conventionalcommits.org/). It is recommended that if you are merging in pull requests regularly that you add a browser extension that will auto-correct the title for you. A few that should do this are [Refined GitHub](https://github.com/sindresorhus/refined-github) and [Squashed Merge Message](https://github.com/zachwhaley/squashed-merge-message).
+
 ### Documenting breaking changes
 
 Make sure to document the migration strategy in a `MIGRATION.md` file in the package(s) that has breaking changes, eg. `packages/fast-components-styles-msft/MIGRATION.md`.