Mention necessary reboot

After setting "USB debugging (security settings)", a reboot is
necessary.

FAQ.md

@@ -159,6 +159,8 @@ In developer options, enable:
 > **USB debugging (Security settings)**  
 > _Allow granting permissions and simulating input via USB debugging_
 
+Rebooting the device is necessary once this option is set.
+
 [simulating input]: https://github.com/Genymobile/scrcpy/issues/70#issuecomment-373286323
 
 

README.md

@@ -47,7 +47,8 @@ Make sure you [enabled USB debugging][enable-adb] on your device(s).
 
 On some devices, you also need to enable [an additional option][control] `USB
 debugging (Security Settings)` (this is an item different from `USB debugging`)
-to control it using a keyboard and mouse.
+to control it using a keyboard and mouse. Rebooting the device is necessary once
+this option is set.
 
 [control]: https://github.com/Genymobile/scrcpy/issues/70#issuecomment-373286323