doc: readme

README.md

@@ -3,3 +3,6 @@ Shell scripts that perform automatic logins for the
 [Travelmate](https://openwrt.org/docs/guide-user/network/wifi/wifiextenders/travelmate) plugin.
 On the device, they live in `/etc/travelmate/*.login` and have to be executable.
 Calling arguments can be used and customized.
+
+## Testing
+`test/functions.sh` is a mock implementation that should be symlinked to `/lib/functions.sh`.