Document release process

RELEASING.md

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+# Releasing addlicense
+
+Releases of `addlicense` are done on an *ad hoc* basis when significant changes
+have been made. If you are unable to run addlicense at head (e.g. it's part of a
+GitHub workflow) and need a recent feature, please file an issue requesting a
+new release.
+
+## Process
+
+The release workflow is configured at
+[.github/workflows/release.yaml](./.github/workflows/release.yaml). A new
+release will be automatically created whenever any tag is pushed to the
+repository. (TODO: consider only matching `v[0-9]*`.) Start a release by adding
+a tag, then push that tag to GitHub:
+
+```sh
+git tag -a v1.2.3-rc0
+git push origin master --tags
+```
+
+The release workflow will start a Go build and create a
+[GitHub release](https://docs.github.com/en/repositories/releasing-projects-on-github/managing-releases-in-a-repository).
+You can check the status of the release workflow and investigate any errors
+[on the Actions page](https://github.com/google/addlicense/actions/).
+
+When the workflow finishes, check the
+[Releases page](https://github.com/google/addlicense/releases) and find the
+release for the new tag. This will have an automatic changelog list. Edit the
+release to highlight any major new features or other items of note. If this is a
+release candidate (`-rc0` suffix on the tag), check the "Set as a pre-release"
+box. Then publish the release! If you are creating a new release after a
+pre-release step, make sure the "Previous tag" is set to the prior main release
+when generating the release notes, e.g. create a list of changes between
+`v1.2.0` and `v1.3.0` rather than between `v1.3.0-rc1` and `v1.3.0`.