docs(conn, key, model): Adding stability index

The GoAngular API will steadily improve as it matures. Certain parts
still need to be refined.  We want to make it clear which state each
part of the API is in, so we'll be leveraging NodeJS's stability index.

More can be found on the
[stability index here](http://nodejs.org/api/documentation.html).
Merge pull request #60 from goinstant/mattcreager/add-stability-index

docs(conn, key, model): Adding stability index