Adding u2f appid extension

Closes Issue #216

index.bs

@@ -1923,6 +1923,34 @@ This section defines an initial set of extensions. These are recommended for imp
 interoperability.
 
 
+## U2F AppId ## {#extension-u2f_appid}
+
+This authentication extension allows [RP]s who have previously registered an
+Authentication using the legacy FIDO U2F JavaScript APIs to communicate with
+Authenticators.  Specifically, this extension allows [RP]s to specify an
+|appId| to overwrite the otherwise computed |rpId|.
+
+: Extension identifier
+:: `webauthn_u2f-appid`
+
+: Client argument
+:: A single UTF-8 encoded string specifying a FIDO U2F |appId|.
+
+: Client processing
+:: AppId validation (as defined by the FIDO U2F specification).  If valid, the
+    value of |rpId| for all Client processing should be replaced by the
+    specified |appId|.
+
+: Authenticator argument
+:: none
+
+: Authenticator processing
+:: none
+
+: Authenticator data
+:: none
+
+
 ## Transaction authorization ## {#extension-txauth}
 
 This authentication extension allows for a simple form of transaction authorization. A [RP] can specify a prompt string,