Update source/reference/operators.txt

Added a note on how to search for MinKey - already added to wiki

Author: comerford

source/reference/operators.txt

@@ -479,7 +479,16 @@ Element
 
    ``MinKey`` and ``MaxKey`` compare less than and greater than all
    other possible :term:`BSON` element values, respectively, and exist
-   primarily for internal use.
+   primarily for internal use.  
+
+   .. note::
+
+   Searching for ``MaxKey`` by type will work as expected, however
+   to search for ``MinKey`` you must use -1, for example:
+
+      .. code-block:: javascript
+
+         db.collection.find( { field: { $type: -1 } } );
 
    .. note::