[LITMUS^RT] change in repository structure --- no more `staging`

[Björn Brandenburg]

Another change going forward is that there will be no more `staging` branch. Instead, we'll have the following:

	1) The master branch, which contains all merged patches on top of the latest release.

	2) prop/ branches (e.g., `prop/topic-1`, `prop/topic-2`, etc.), which are topic branches containing change proposals.

	3) When the time comes, `next` will contain the rebased version that will become the next stable release of LITMUS^RT.

To stay up-to-date, follow the master branch. New major changes will be staged in prop/ branches and merged into master when ready. Prior to the next release, LITMUS^RT will be squashed and rebased in `next`. When a new release is made, `master` will be reset.

In short, `staging` is gone. The rationale for this change is that the difference between `staging` and `master` wasn't really clear anymore, and that we were not deriving substantial benefits from keeping the two apart.

Thanks,
Björn