Added a todo item.

Change-Id: Iefe69577980a6fe37e2b400303064686cf1e7b3a
diff --git a/model/src/main/java/org/onosproject/yang/model/YangModel.java b/model/src/main/java/org/onosproject/yang/model/YangModel.java
index 74dd0cb..4d50b16 100644
--- a/model/src/main/java/org/onosproject/yang/model/YangModel.java
+++ b/model/src/main/java/org/onosproject/yang/model/YangModel.java
@@ -52,5 +52,9 @@
      * @param id     module identifier
      * @param module YANG module information
      */
+    @Deprecated
     void addModule(YangModuleId id, YangModule module);
+
+    // TODO: Make this class immutable post-ONS.
+    // Either specify modules during construction or create a builder.
 }