Add list of supported extensions to README

README.md

@@ -95,6 +95,13 @@ server.listen(80);
 
 A more elaborate example including utilizing extensions can be found in `sample.js` in the GitHub repository.
 
+## Supported Extensions
+This library comes with built-in support for the following OpenID extensions:
+- The Simple Registration (SREG) 1.1 extension is implemented as `openid.SimpleRegistration`
+- The Attribute Exchange (AX) 1.0 extension is implemented as `openid.AttributeExchange`
+- The OAuth 1.0 extension is implemented as `openid.OAuthHybrid`
+- The User Interface 1.0 extension is implemented as `openid.UserInterface`
+
 ## Storing association state
 
 To provide a way to save/load association state, you need to mix-in two functions in