Improve contributing guidelines

As we're having more people contributing it's exposing some of the inadequacies of the CONTRIBUTING.md

A particular source of this is

We are keen to maintain a high standard of the code.

We really need to put together a good set of links or specific instructions on style guidelines.

Things we should cover:

@gregory.ashton @moritz.huebner @matthew-pitkin do you have any other things to add to the list above?