Added Plex to the list of supported sites in README.
Added Plex to the list of supported sites in README.

vladikoff/keysocket,Whoaa512/keysocket,kristianj/keysocket,borismus/keysocket,feedbee/keysocket,ALiangLiang/keysocket,vinyldarkscratch/keysocket,feedbee/keysocket,legionaryu/keysocket,vinyldarkscratch/keysocket,chrisdeely/keysocket,borismus/keysocket,iver56/keysocket,noelmansour/keysocket,kristianj/keysocket