Add Extension Registry URL to spec.

Similar to Micropub, the specification should link to where proposed extensions to IndieAuth may be cited...such as supporting oauth2 extensions and the differences needed to implement them in an IndieAuth server if they aren't going to be added to the main specification.

Start with the IndieAuth specification and compare how Micropub links to proposed extensions. Locate the specification entry point in the repository, then add a link to an extension registry covering OAuth2 extensions and implementation differences; done means the registry is clearly linked from the specification.

Written by the indexing model from the issue text.