Amend `RELEASING.md`

[DigitalBrains1]

Add Mergify instructions and improve the instructions for writing CHANGELOG.md.

Still TODO:

• Write a changelog entry (see changelog/README.md)
• [x] Check copyright notices are up to date in edited files

[DigitalBrains1]

Erm, I think when you look at development process documentation in a git repo, you look at master. If we say you can also look in stable branches, we'd have to backport it to every stable branch we might conceivably ever do another release for, every time. Ideally, the stable branches wouldn't even have the file to begin with, but I think that's a bit more trouble than it's worth.

So I'd rather not backport this so as not to create false hopes that somehow outdated development process documentation is still valid.

[martijnbastiaan]

Fair enough!