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

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 :sweat_smile:

- 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

Open the CONTRIBUTING file and add a release section based on the checklist in the issue. Ensure it covers drafting the release, optional screencapture GIFs, emojis, pre-release marking, version changes in setup.py, and the vX.Y.Z tag; done means each listed step is documented.

Written by the indexing model from the issue text.