Add documentation for implemented identity meters

Add a list of Identity events they can be metered, specifically:

- user authentication
- project create/update/delete
- role create/update/delete
- user create/update/delete
- group create/update/delete
- trust create/delete

Change-Id: I7a2f25c9e5b64e8f3a2ff986660e5ed531d955c3
Merge "Add documentation for implemented identity meters"