Improve command line help

Fixes #450

src/main/java/org/asamk/signal/commands/AddDeviceCommand.java

@@ -22,6 +22,7 @@ public class AddDeviceCommand implements LocalCommand {
 
     @Override
     public void attachToSubparser(final Subparser subparser) {
+        subparser.help("Link another device to this device. Only works, if this is the master device.");
         subparser.addArgument("--uri")
                 .required(true)
                 .help("Specify the uri contained in the QR code shown by the new device.");