add note about user to run service as

README.md

@@ -51,6 +51,9 @@ Install bubble-flexrouter as a system service (Windows Service or Mac OS launch
 
 It should always be running. Set it to run at system startup.
 
+The user that bubble-flexrouter runs as must have sufficient privileges to add and remove IP routes from the
+system routing table. This usually means Administrator on Windows or root on Mac OS.
+
 The service requires some environment variables to be set:
 
  * `BUBBLE_FR_SSH_KEY` - full path to the *private* SSH key