add to upgrade guide

Author: norareidy

docs/upgrade.txt

@@ -74,6 +74,11 @@ This library version introduces the following breaking changes:
 - In query results, the library converts BSON ``UTCDateTime`` objects to ``Carbon``
   date classes, applying the default timezone.
 
+- ``id`` is an alias for the ``_id`` field in MongoDB documents, and the library
+   automatically converts between ``id`` and ``_id`` when querying data. Because
+   of this behavior, you cannot have two separate ``id`` and ``_id`` fields in your
+   documents.
+
 .. _laravel-breaking-changes-v4.x:
 
 Version 4.x Breaking Changes