add command to install repo key

README.md

@@ -28,6 +28,7 @@ this line in `/etc/apt/sources.list.d/autoradio.list`:
 
 And then running:
 
+    $ sudo apt-key adv --recv-key 0xC0EAC2F9CE9ED9B0
     $ sudo apt-get update
     $ sudo apt-get install etcd autoradio