Document release process

[grodin]

It would be useful to document the release process, for any future maintainer's sakes and also to make it clearer about things like generating docs, whether they should be part of the release or part of a PR.

An obvious further step would be to automate the release process, but I don't know how involved it is and how easy that would be.

[evant]

Here's the basic steps just to have something down:

1. submit a release pr, this should:
   a. remove SNAPSHOT from the version
   b. update the CHANGELOG to include the version and the release data, verify that changes listed are correct
   c. update the version listed in the README
   d. run ./gradlew generateDocs to update the docs
2. after that is merged, wait for ci to build and publish, then go to https://oss.sonatype.org and release it
3. tag the release, create a release on github from the tag, copy in the changes listed in the CHANGELOG
4. submit a new pr that bumps the version and adds SNAPSHOT back