Document how to release IRB [ci skip]

This is what hsbt has been using these days.

Author: k0kubun

README.md

@@ -105,6 +105,13 @@ To install this gem onto your local machine, run `bundle exec rake install`. To
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/irb.
 
+## Releasing
+
+```
+rake release
+gh release create vX.Y.Z --generate-notes
+```
+
 ## License
 
 The gem is available as open source under the terms of the [2-Clause BSD License](https://opensource.org/licenses/BSD-2-Clause).