Labeling current versions

[jimfenton]

Both the mongodb-engine and the djangoappengine installation pages reference the 1.5 branch from GitHub. From what I can tell and based on a response on the mailing list, 1.6 is stable now, and if so that should be the recommended version.

More generally, the documentation somewhere should say something about the stability of the various version branches. Apparently 1.7 isn't "ready for prime time" yet but that should be more clearly stated somewhere.