Add "how to release a version of Empress" developer documentation to CONTRIBUTING file

[fedarko]

Yours truly forgot to update the version number in setup.py before doing the release, leading to some last-minute panic as we had to redo the release. It'd be nice to have this process written down somewhere so that we don't have to deal with these problems again, and so that future generations of Empress devs have information on how to do this 😅

• Mention drafting the release
• Mention adding screencapture GIFs, if applicable
• Mention adding obligatory emojis :)
• Mention marking as a pre-release (for now)
• Mention bumping version number in setup.py to X.Y.Z before cutting the release
• Mention setting the tag name to vX.Y.Z
• Mention bumping version number in setup.py to X.Y.Z-dev after cutting the release
• anything i'm forgetting here