Move contributing from hacking docs to repo root

A CONTRIBUTING.rst is added to the root of the repo, as Github
automatically recognizes this for incoming PRs
(ref: https://github.com/blog/1184-contributing-guidelines)

Most of the information is directly borrowed from the existing hacking
docs, which is restructured to refer to this file instead for
contribution guidelines
Merge pull request #587 from theanalyst/docs/contributing

Move contributing from hacking docs to repo root