document how to create a release using the contents of a file

[kne42]

Document in the README example how to use the contents of a file as the body of the release. I chose to replace the static example with one that reads from a file as I do not believe it is likely that many people will want the release notes to be the same every time.

I additionally chose to include this example as it is surprisingly non-trivial to read the contents and pass them on to the action as one will need to know how to use workflow actions and that multi-line strings need to be escaped. This points them to an action that will handle everything for them.