Add release notes

RELEASE.md

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+# Releasing buf-action
+
+This document outlines how to create a release of buf-action.
+We follow the best practices outlined in the [GitHub Actions documentation][github-release-docs] and [GitHub Actions Toolkit tutorial][github-release-tutorial].
+
+1. Clone the repo, ensuring you have the latest main.
+
+2. On a new branch, run the following commands to update the version of the action:
+
+```bash
+VERSION=X.Y.Z make update-version
+```
+
+3. Open a PR titled "Prepare for vX.Y.Z". Once it's reviewed and CI passes, merge it.
+
+    *Make sure no new commits are merged until the release is complete.*
+
+4. Create a new release using the Github UI:
+    - Under “Choose a tag”, type in “vX.Y.Z” to create a new tag for the release upon publish.
+    - Target the main branch.
+    - Title the Release “vX.Y.Z”.
+    - Click “set as latest release”.
+    - Click "Publish this Action to the GitHub Marketplace".
+
+
+5. Publish the release.
+
+6. Pull down the latest main and move the tags for major (vX) and minor (vX.Y) releases.
+
+```bash
+git tag -fa vX -m "Update vX tag"
+git tag -fa vX.Y -m "Update vX.Y tag"
+git push origin vX vX.Y --force
+```
+
+[github-release-docs]: https://docs.github.com/en/actions/creating-actions/releasing-and-maintaining-actions
+[github-release-tutorial]: https://github.com/actions/toolkit/blob/master/docs/action-versioning.md