Credentials.user_id may not be NULL and README

I received the error `SQLite3::ConstraintException: credentials.user_id may not be NULL: INSERT INTO "credentials ("credentials") VALUES(?) " ` on line `user.create_credentials! :credentials => auth.credentials` and i fixed inserting `:user => user, `.

The `README` have some inconsistency between the files, the function `update_credentials` doesn't exists anymore. 
maybe cool update it?