Add "Accepting a GitHub Pull Request" protocol

> Possible failures I've seen
> at or around this step
> include
> having committed to local master but not pushed that
> so that there is a divergence between origin/master and master
> and a commit being pushed to origin/master

I agree, this was very poetic.

The error message we get in that scenario is pretty self-explanatory and easy to recover from.

`--ff-only` matches our standard Git protocol:

https://github.com/thoughtbot/guides/tree/master/protocol/git#merge

> Since this is a guide not only for thoughtbot employees
> who might be able to solve that
> but also referenced by beginners
> those points of failure might be worth addressing.

Maybe for a blog post? I'm not sure it fits in the guides.