Update readme to for autopilot opt-in

Author: tanx

README.md

@@ -76,7 +76,7 @@ To run the packaged version of the app e.g. for macOS run:
 
 The app is configured for testnet by default but you can opt-in to mainnet if you have a btcd node running. Be aware that this is currently still experimental:
 ```
-./dist/mac/Lightning.app/Contents/MacOS/Lightning --bitcoin.mainnet --bitcoin.node=neutrino --neutrino.connect=127.0.0.1:8333
+./dist/mac/Lightning.app/Contents/MacOS/Lightning --bitcoin.mainnet --bitcoin.node=neutrino --neutrino.connect=127.0.0.1 --autopilot.active
 ```
 
 ### Starting the Packaged App (full node)