Document how to try out a pull request in a virtualenv

I would like to try out the pull request #332 without messing with my regular astropy installation.
For non-power users of git, github and virtualenv this is non-trivial and it would be nice to have the commands to do it listed in one place.

There's already documentation at
http://docs.astropy.org/en/latest/development/workflow/maintainer_workflow.html
on how to get and check out the branch of interest, and at
http://docs.astropy.org/en/latest/development/building_packaging.html
on how to create a virtualenv and install that astropy branch in it.

I guess all I'm asking is if it would't be worth it to include a section in
http://docs.astropy.org/en/latest/development/workflow/development_workflow.html
on "How to try out a pull request" that puts this information together, something like
```
cd <your astropy repo>
git remote add someone git://github.com/someone/astropy.git
git fetch someone
git branch cool-feature --track someone/cool-feature
git checkout cool-feature
mkdir virtualenvs # does this work or does the folder have to be outside the repo?
cd virtualenvs
virtualenv --system-site-packages cool-feature
source cool-feature/bin/activate
cd ..
python setup.py install
# now you can try out the pull request
# also show the commands to delete the virtualenv
# remove the remote and branch to clean up
```

Note that ipython has https://github.com/ipython/ipython/blob/master/tools/test_pr.py which might be a nice addition to astropy.