templates: add release-checklist

This collects all release steps in a release-checklist template.

docs/CONTRIBUTING.md

@@ -177,3 +177,9 @@ Instead do this:
 ## Contributing Tutorial
 
 For a detailed walk-through on building, modifying, and testing, see this [tutorial on how to start contributing to OSTree](contributing-tutorial.md).
+
+## Release process
+
+Releases can be performed by [creating a new release ticket][new-release-ticket] and following the steps in the checklist there.
+
+[new-release-ticket]: https://github.com/ostreedev/ostree/new?labels=kind/release&template=release-checklist.md