Update release instructions

These aren't fully correct, since direct push is currently not
allowed.

The main change here is switching the release announcement from
GitHub releases to GitHub Discussions, to encourage community
feedback. Approximately nobody is downloading from the releases
page, everyone is installing via rubygems.

README.md

@@ -128,12 +128,12 @@ Contributions are welcome. Please make sure to:
 
 ## Releasing
 
-* Ensure `History.md` is up-to-date
 * Bump VERSION in lib/rack/test/version.rb
+* Ensure `History.md` is up-to-date, including correct version and date
 * `git commit . -m 'Release $VERSION'`
 * `git push`
 * `git tag -a -m 'Tag the $VERSION release' $VERSION`
 * `git push --tags`
 * `gem build rack-test.gemspec`
 * `gem push rack-test-$VERSION.gem`
-* Update the [GitHub releases page](https://github.com/rack/rack-test/releases)
+* Add a discussion post for the release