Add developer policy about contractual APIs

Document what APIs nova considers to be public contracts, anything not
in this list should be considered a private API.

Clarify the 'API' section to make sure its clear we are talking about
the REST API.

Change-Id: If71adc7a9d5dc1bde79f75d10e67500f6f92b81f
Merge "Add developer policy about contractual APIs"