Document proposed new triangular workflow and developer certificate of origin

We plan to move the bnd project to use the triangular workflow and the developer certificate of origin in about 4 weeks. The details are in the CONTRIBUTING.md document.

This means that all contributions must come in via a pull request from a forked repo. Maintainers must no longer directly push to the bndtools/bnd repo. Instead they fork the repo and push their commits to the fork and then a pull request is made to merge them into the bndtools/bnd repo.

It also means that all commits must have a `Signed-off-by` statement at the end of the commit message to indicate the committer agrees to the [Developer Certificate of Origin](http://developercertificate.org/).