document keystone-specs instead of LP blueprints in README

Change-Id: I1d6c97e5f6033c530baa8bb3e24c911f4b57a824
Add v2 & v3 API documentation

Keystone itself didn't have much documentation on the APIs it was
implementing, so I made space for that documentation first, and then
followed it with a first pass on what it takes to move things to v3.

Change-Id: I71ee6d3c87b6919eb2f7569069d91f3a00d6337c
bp: document-v2-to-v3-transition
Merge "Add v2 & v3 API documentation"
Merge "document keystone-specs instead of LP blueprints in README"