Add more documentation about multiple release channel approaches

[JAForbes]

No description provided.

[pygy]

I suppose having a --changelog-branch option would make sense here.

[JAForbes]

A fine idea, but I really think this library shouldn't operate at that level of abstraction. The key thing is pr-release makes it easy to access the metadata of the release so each project can do this their way. Until I see lots of projects do the same thing I don't want to complicate this tool.

# get all the metadata about the release
npx pr-release extract-changelog --out

# Use it in some project specific script
node scripts/update-main-changelog ./.pr-release/changelog.md

In mithril's case, I think we should just keep the changelog's on their own release channel, and glue them together in the docs build for the html output only. Keep the changelog markdown separate for each channel because they are separate histories. But that's my view, maybe others want to do this cross branch updating thing, and that's fine. But I don't think pr-release should have it as built in behaviour.