add ExtensionType interface
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 29b8268..2f997ae 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -909,6 +909,13 @@
 
         Returns an instance of the extension type corresponding to the OID.
 
+.. class:: ExtensionType
+
+    .. versionadded:: 1.0
+
+    This is the interface against which all the following extension types are
+    registered.
+
 .. class:: KeyUsage
 
     .. versionadded:: 0.9