A command line workflow for adding just source/doc entries to rosdistro without a bloom release.

[nuclearsandwich]

Since we're making adjustments to the requirements for ros2-gbp repositories (ros2-gbp/ros2-gbp-github-org#106) it's going to be a much more common case that folks will want to add their repository with just a source or source and doc entries before a first release.

Whether bloom is the right tool for that remains to be seen but since it has most of the facilities that are required to make it happen I'm starting the issue here in and we can move it elsewhere later on.