Add info about getting underscored root object in create/update requests.

In 0.13 the REST Adapter would send underscored versions of multi word
model names as the root element.  Beginning in Beta 1 the REST Adapter
sends camelized root object.

This PR add info to TRANSITION.md about how to get back to underscored root objects.
Merge pull request #1250 from jagthedrummer/jeremy/underscore-root-json-serializer

Add info about getting underscored root object in create/update requests