minor mongofiles will not read from the secondary

Author: Sam Kleinman

source/reference/mongofiles.txt

@@ -30,6 +30,11 @@ Like :program:`mongodump`, :program:`mongoexport`,
 access data stored in a MongoDB data directory without requiring a
 running :program:`mongod` instance, if no other :program:`mongod` is running.
 
+.. note::
+
+   For :term:`replica sets <replica set>`, :program:`mongofiles` can
+   only read from the set's ':term:`primary`.
+
 .. _mongofiles-commands:
 
 Commands