Add initial guide to using pull requests

This guide is intended to help us write better pull requests, and to be
better reviewers when helping to merge things.

It's basically complete as-is but could do with some examples.
Pull requests: cross-linking related PRs
Make pairing/2nd pair of eyes distinction explicit

It was pointed out that this item was unclear under pairing
circumstances - we need an 'independent' 3rd party to do the merge, so
pairs shouldn't merge each other's work.
Add line about running code
Add extra section on praising commits
Merge pull request #45 from alphagov/pull-requests

Add initial guide to using pull requests