Update documentation for Oauth2 provider in Rails 2.x

Hello,

I worked a lot to understand how to integrate the gem with my Rails 2.3 and I figured out that the documentation did not specify to add the rack filter for Rails 2.x versions

I just fixed it and also add the oauthenticate method as example for action protection.

Regards,

Signed-off-by: Yannick Chaze <yannick@wearecloud.com>