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
while `bundle` and `rake` and any possible fixes take place.

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.