Add "Accepting a GitHub Pull Request" protocol

Hub does provide a shortcut for checking out pull requests.

```
guides@master / % git checkout https://github.com/thoughtbot/guides/pull/209
error: pathspec 'https://github.com/thoughtbot/guides/pull/209' did not match any file(s) known to git.
guides@master / % hub checkout https://github.com/thoughtbot/guides/pull/209
Updating thoughtbot
remote: Counting objects: 17, done.
remote: Compressing objects: 100% (16/16), done.
remote: Total 17 (delta 5), reused 3 (delta 0)
Unpacking objects: 100% (17/17), done.
From git://github.com/thoughtbot/guides
 * [new branch]      dc-open-source -> thoughtbot/dc-open-source
Branch thoughtbot-dc-open-source set up to track remote branch dc-open-source from thoughtbot.
Switched to a new branch 'thoughtbot-dc-open-source'
```

If you alias `hub` as `git`, you can use either command, but Git doesn't seem to recognize pull request URLs on its own.