README: mention that password is needed for MacOS built-in client

Closes #8

Author: bk138

README.md

@@ -40,6 +40,9 @@ In its default setup, macVNC does mouse and keyboard input. For this, it needs c
 It tells you on first run if these are missing; you can set up permissions via 'System Preferences'->'Security & Privacy'->'Privacy'->'Accessibility'.
 Note that if launched from Terminal, the entry shown will be 'Terminal', not 'macVNC'.
 
+Note that setting a password is mandatory in case you want to access the server using MacOS's built-in Screen Sharing app.
+You can do so via the `-passwd` commandline argument.
+
 # License
 
 As its predecessors, macVNC is licensed under the GPL version 2. See [COPYING](COPYING) for more information.