update readme with session and provider interface links

Author: Mohan Sekar

README.md

@@ -4,7 +4,7 @@ Package goth provides a simple, clean, and idiomatic way to write authentication
 packages for Go web applications.
 
 Unlike other similar packages, Goth, lets you write OAuth, OAuth2, or any other
-protocol providers, as long as they implement the `Provider` and `Session` interfaces.
+protocol providers, as long as they implement the [Provider](https://github.com/markbates/goth/blob/master/provider.go#L13-L22) and [Session](https://github.com/markbates/goth/blob/master/session.go#L13-L21) interfaces.
 
 This package was inspired by [https://github.com/intridea/omniauth](https://github.com/intridea/omniauth).