docs: add github-merge.py process for merging PRs

[delta1]

Documents how to merge PRs using the github-merge.py script from Bitcoin maintainer tools repo.

[apoelstra]

In c73be55:

I'd suggest downloading it to $HOME/bin/ rather than into your repo where you have to gitignore it. (Actually, I have the whole maintainer-tools repo cloned, a separate workspace for the "in use" commit, and a symlink from there to my ~/bin.)

But I can see the merits to either approach.

[delta1]

Thanks that’s a good suggestion, I’ll try that out and update the doc