add CONTRIBUTING.md

[minrk]

GitHub just added support for a CONTRIBUTING[.md] file, which will be shown to users opening an Issue or Pull Request. This might be helpful for improving the quality of initial bug reports / pull requests, assuming it is sufficiently prominent and concise.

It should include a quick summary of things like:

• search the Issues before opening a new one
• If possible, test with current master before the reporting a bug that may already be fixed
• For bugs, include the versions of relevant pacakges, and the output of print IPython.sys_info()
• first places to look for prospective new contributors (quickfix / docs tags, PR review)
• Pull Request guidelines, etc.

[ellisonbg]

Ohh, this is awesome, we should definitely do this. BUT let's keep in mind that we have developer docs spread all over the place already. Towards the end of the summer we had talked about consolidating our development related docs/wiki stuff to the github wiki. I think we even took an informal poll, but I don't recall if we actually concluded it.