Add initial guide to using pull requests

Add initial guide to using pull requests

This guide is intended to help us write better pull requests when working on GOV.UK, and to be better reviewers when helping to merge things.

It's basically complete as-is but could do with some examples.

This was initially checked over by @evilstreak, @alext, @dcarley, @rjw1, @dsingleton, and @jamiecobbett.

I'd appreciate reckons from @fatbusinessman and @alicebartlett at least.