RELEASE_PROCEDURE.md

Non-major versions go via release-plz.

1. bumps the version number.

   If there are breaking changes, the major version number should be increased.

2. If the major version number gets bumped, update SECURITY.md.

3. Overwrite BREAKINGCHANGES with BREAKINGCHANGES_dev, and create a new dev file:

   $ cd topgrade
   $ mv BREAKINGCHANGES_dev.md BREAKINGCHANGES.md
   $ touch BREAKINGCHANGES_dev.md