Guide for contributors

We need to create this. The one at http://readthedocs.org/docs/packaginator/en/latest/contributing.html looks pretty close to what we will want, though we'll probably need to add something about signing your commits and the contributory agreement.

One thing that is mentioned there that I think we need to be aware of is

Don't mix whitespace cleanup changes with other commits.

It makes it much harder to properly review a change if mixed in with the real change there are lots of other purely cosmetic changes to the file - reformatting, removal of commented-out blocks, removal of trailing whitespace etc. While continually improving the sources is generally a good idea, it is best done via separate pull requests rather than intermixed with 'real' changes.

(I'm not sure what the big objection to trailing whitespace is, to be honest. Sure, all things being equal I'd rather it was not there but whether it justifies the pain of removal on existing files I'm not sure...)