Document that this is new in 0.2
diff --git a/docs/hazmat/primitives/interfaces.rst b/docs/hazmat/primitives/interfaces.rst
index 12644f4..2adad91 100644
--- a/docs/hazmat/primitives/interfaces.rst
+++ b/docs/hazmat/primitives/interfaces.rst
@@ -209,6 +209,8 @@
 
 .. class:: KeyDerivationFunction
 
+    .. versionadded:: 0.2
+
     .. method:: derive(key_material)
 
         :param key_material bytes: The input key material. Depending on what