[Biopython] Draft CONTRIBUTING file, e.g. for GitHub Pull Requests

[Peter Cock]

Hello all,

GitHub has popularised including a CONTRIBUTING text
file in the root of a project repository, which they will show
to people preparing a pull request:

https://help.github.com/articles/setting-guidelines-for-repository-contributors/

I've prepared a draft CONTRIBUTING file as part of this pull request:

https://github.com/biopython/biopython/pull/904

Some extra eyes on this would be most appreciated.

Thanks,

Peter