add release docs (#388)

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

Author: wagoodman

RELEASE.md

@@ -0,0 +1,19 @@
+# Release
+
+A release of scan-action comprises:
+- a new semver git tag from the current tip of the main branch
+- a new [github release](https://github.com/anchore/scan-action/releases) with a changelog
+- the action distributable committed into the repo at `dist/`
+
+Ideally releasing should be done often with small increments when possible. Unless a
+breaking change is blocking the release, or no fixes/features have been merged, a good
+target release cadence is between every 1 or 2 weeks.
+
+
+## Creating a release
+
+Releases are automatically drafted on every push to the main branch. Please see the [github releases page](https://github.com/anchore/scan-action/releases) for the latest draft. To publish the release:
+
+- Click "edit" (the pencil icon)
+- Modify the changelog as needed (for instance, if grype was bumped multiple times, include only the latest version bump entry)
+- Click "publish"