Add branchless tempest section to README.rst

This commit adds a section to the tempest README outlining some things
that have to be watched when pushing changes to tempest because of
branchless tempest

Change-Id: I9dbf042cef6af51c92d48d42ba04c1582690bda7
Add README section about API stability

This commit adds a section to the README outlining that tempest tests
should only be added for stable APIs. If the API is tested the
expectation is that it complies with the API stability guidelines.

Change-Id: I02a93f66ff45e4d687b7395fb24ee5bda6e52fb0
Merge "Add branchless tempest section to README.rst"
Merge "Add README section about API stability"