docs: document NATIVE_SPEEDTEST_PATH config option

- Added details for NATIVE_SPEEDTEST_PATH to the README under 'Usage'.
- Explained default behavior and included examples for overriding the binary location.
- Added a verbose log to print the binary path when the plugin starts up.

front/plugins/internet_speedtest/script.py

@@ -50,6 +50,7 @@ def main():
 
 def run_speedtest():
     native_path = NATIVE_SPEEDTEST_PATH
+    mylog('verbose', [f"[INTRSPD] Using native binary path: {native_path}"])
     if os.path.exists(native_path):
         mylog('verbose', ["[INTRSPD] Native speedtest binary detected, using it."])
         try: