Add Authorization section to README (follow the new format)

[jgeewax]

See GoogleCloudPlatform/gcloud-meta#3 for what we're hoping to be the shared (mostly) format.

[dhermes]

I'm updating the title based on my comment "Holding off on Authorization until we finalize this behavior and unify across services." in #673.

[dhermes]

@jgeewax What do we want to do about this?

[jgeewax]

We obviously need extra stuff outside of the shared docs, but anything that is common would (ideally) get pushed back up into gcloud-common... Otherwise, we can call this done.