Update README to include note for oauth token.

"Note" is now a required parameter for creating an authorization.
https://developer.github.com/v3/oauth_authorizations/#create-a-new-authorization
Merge pull request #471 from mscoutermarsh/master

Update README to include note for oauth token.
No authentication required for user_keys
Merge pull request #474 from robbertkl/patch-1

No authentication required for user_keys