Add upgrade note

Author: alcaeus

UPGRADE-2.0.md

@@ -6,3 +6,9 @@ UPGRADE FROM 1.x to 2.0
    and `getPort()` methods have been added in its place.
  * The BSON functions in the `MongoDB\BSON` namespace have been removed in favor 
    of the `MongoDB\BSON\Document` class.
+ * All tentative return types defined in interface and non-final classes are now
+   fixed and are required in implementing or extending classes.
+ * `MongoDB\Driver\CursorInterface` now extends `Iterator`, requiring 
+   implementing classes to also implement iterator methods. The return types for
+   the `key` and `current` methods have been narrowed to the types returned by
+   cursor instances.