Add configuring docs for oc-id.

The CHEF_OAUTH2_SITE docs are a little vague because it is not intended to be
used. It could be removed, as I do not think that other organizations using
Supermarket would standup their own OAuth2 provider that replicates oc-id, but
you never know.
Merge pull request #257 from opscode/doc_oc-id_config

Add configuring docs for oc-id.