Document new triangular workflow and developer certificate of origin

We are moving the bnd project to use the triangular workflow and the developer certificate of origin. The details are on 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.