Add guidelines for providing contributions

* Thanks to @mhaskel for providing the basis of this
Should we add stuff about dialogue?

```md
 5. Responding to feedback.

    We may have feedback for you to fix or change some things. We generally like to see that pushed against the same topic branch (it will automatically update the Pull Request). You can also fix/squash/rebase commits and push the same topic branch with `-force` (it's generally acceptable to do this on topic branches not in the main repository, it is generally unacceptable and should be avoided at all costs against the main repository). 

  The only reasons a pull request should be closed and resubmitted are as follows:

      * When the pull request is targeting the wrong branch (this doesn't happen as often).
      * When there are updates made to the original by someone other than the original contributor. Then the old branch is closed with a note on the newer branch `This supersedes #github_number`.

```