🧹 Update documentation (#147)

- Document the release process in `CONTRIBUTING.md`
- Add development instructions to `CONTRIBUTING.md` (adapted from [download-artifact](https://github.com/actions/download-artifact))
- Add a test badge to the `README`
- Link to `CONTRIBUTING.md` in `README`
- Unify copy between `README.md` and `action.yml`
- Copy-pasta the CoC from [download-artifact](https://github.com/actions/download-artifact)

README.md

@@ -1,6 +1,12 @@
 # Pull Request Labeler
 
-Pull request labeler triages PRs based on the paths that are modified in the PR.
+<p align="left">
+  <a href="https://github.com/actions/labeler/actions?query=workflow%3A%22Build+%26+Test%22++">
+    <img alt="build and test status" src="https://github.com/actions/labeler/actions/workflows/build_test.yml/badge.svg">
+  </a>
+</p>
+
+Automatically label new pull requests based on the paths of files being changed.
 
 ## Usage
 
@@ -104,3 +110,7 @@ Various inputs are defined in [`action.yml`](action.yml) to let you configure th
 | `repo-token` | Token to use to authorize label changes. Typically the GITHUB_TOKEN secret | N/A |
 | `configuration-path` | The path to the label configuration file | `.github/labeler.yml` |
 | `sync-labels` | Whether or not to remove labels when matching files are reverted or no longer changed by the PR | `false`
+
+# Contributions
+
+Contributions are welcome! See the [Contributor's Guide](CONTRIBUTING.md).