7.10.0: Document release automation

[marcustyphoon]

Adds documentation about the (convoluted... but fewer steps than previously?) XKit version release process automation.

I think the worst thing about this is currently the xkit_patches version bump, which I managed to screw up earlier today. If anyone can think of a way to refactor run_order so this doesn't have to be done or can be simpler, I'm all ears (just make sure your idea works with mismatched xkit_patches and xkit versions). Removed as of #2165 and #2166!

[marcustyphoon]

Might make sense to add

EXTENSION_ID=
CLIENT_ID=
CLIENT_SECRET=
REFRESH_TOKEN=

to .env.example with a comment link to https://github.com/fregante/chrome-webstore-upload-cli#setup. I don't actually know if that cli tool reads the .env file, though?