Add upgrade note for removed BSON functions

Author: alcaeus

UPGRADE-2.0.md

@@ -4,3 +4,5 @@ UPGRADE FROM 1.x to 2.0
  * The `getServer()` method has been removed from the CommandFailedEvent,
    CommandStartedEvent, and CommandSucceededEvent event classes. The `getHost()`
    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.