Create RELEASING.md

Contents from https://github.com/python-pillow/Pillow/wiki/Release-Checklist and some links added.

I think it would be better to keep the release checklist as part of the repo, rather than on a wiki, as this keeps all our bits and bobs together in case we ever did want to migrate them, and also makes them less hidden.

In https://github.com/python-pillow/Pillow/issues/853 @djc suggested using a RELEASING file in the repo root, and this is a fairly common practice (Google: `site:github.com inurl:RELEASING`).

If accepted, I'll delete the wiki page.

[CI skip]