Document authentication plugins

Explain the plugins that are provided and some information on how to
write new ones. This will obviously need to be expanded as more features
are added.

Change-Id: Ie67e93f2ed994c3702ac66e388cf216609dd44ff
add deprecation warning for auth_token

Change-Id: I1faaae0373a7ae12ee528ab5c71741a2b1122d43
Closes-Bug: 1342274
Merge "Document authentication plugins"
Merge "add deprecation warning for auth_token"