add note about manual file to readme

README.md

@@ -30,6 +30,12 @@ Install
 directory called contextmenu and place the files in there, ignoring the root
 directory in the downloaded archive.
 
+Contextmenu manual
+------------------
+The MANUAL.md file shipped with this plugin contains information for plugin and
+skin developers. The Contextmenu plugin can be extended by other plugins; new
+menus can be created and existing menus manipulated.
+
 [rcplugrepo]: https://plugins.roundcube.net/packages/johndoh/contextmenu
 [releases]: https://github.com/johndoh/roundcube-contextmenu/releases
 [gpl]: https://www.gnu.org/licenses/gpl.html