Change build workflow to take care of creating entire release

RELEASE.md

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+# RGL releases
+
+The RGL does not use semantic versioning.
+Releases are instead made periodically, as snapshots of the current state of the library.
+Releases are Git tagged `YYYYMMDD`, and for each release a binary package (as `.gfo` files) is made available as a GitHub release.
+
+## Creating a new release
+
+1. Run the "Create release" workflow through the GitHub actions interface
+   (instructions [here](https://docs.github.com/en/free-pro-team@latest/actions/managing-workflow-runs/manually-running-a-workflow)).