Add CONTRIBUTING.rst file

Move doc/contributing.rst and doc/contributing-docs.rst into
/CONTRIBUTING.rst, now it should show up on GitHub:
https://github.com/blog/1184-contributing-guidelines
Attempt to fix contributing to docs link
Fix a typo
Merge branch 'master' of github.com:okfn/ckan into 268-datastore-setup-instructions-dont-work
Merge branch '268-datastore-setup-instructions-dont-work' of github.com:okfn/ckan into 268-datastore-setup-instructions-dont-work