Add upgrade note

Author: alcaeus

UPGRADE-2.0.md

@@ -9,3 +9,9 @@ UPGRADE FROM 1.x to 2.0
  * The constants `MongoDB\Driver\ClientEncryption::ALGORITHM_RANGE_PREVIEW` and
    `MongoDB\Driver\ClientEncryption::QUERY_TYPE_RANGE_PREVIEW` have been 
    removed. Use the `ALGORITHM_RANGE` and `QUERY_TYPE_RANGE` instead.
+ * 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.