Document stable branches in devtest
Document stable branches in devtest

This change adds a warning about stability and the tripleo stable branches to
devtest. Anyone wanting to make use of the stable branches should heed the
warning and make sure they are using the correct docs and branches.

Change-Id: I23989fec0f9bcbbc1e47ae6eb2617a8c85afa4ec

rdo-management/tripleo,rdo-management/tripleo