Repos in an organization with no CONTRIBUTING.md use organization wide version

[Peter Roth]

We have many repos and even requiring a file with an import statement is difficult to maintain. I'd like to setup one global organization CONTRIBUTING.md file that all repos default to use.

Perhaps they can override this via a local repo CONTRIBUTING.md (or both versions are shown in PRs somehow) but that is a nice to have not a requirement.

Note that while https://bitbucket.org/atlassianlabs/prguidelines/issues/6/allow-fetching-from-a-single-repo allows central management of the file it still requires changes in ALL repos that use this. This isn't sustainable by hand when there are more than 50 repos in an organization and new ones being created regularly.