Add a note on secure mode

README.md

 
 ## Secure mode
 
+_tl;dr - use secret mode for all new installation of proxy; only clients
+with dd-secrets will be able to connect. This mode abuses attempts to
+DPI MTPROTO traffic._
+
 Secure mode is not the best name and of course, it creates a lot of
 confusion. To explain what it means, we need to tell you some bits on
 dd-secrets.