Add note about service worker usage netlify#2

Author: lgraubner

README.md

@@ -37,3 +37,7 @@ yarn start
 This will start the normal create-react-app dev server and open your app at `http://localhost:3000`.
 
 Local in-app requests to the relative path `/.netlify/functions/*` will automatically be proxied to the local functions dev server.
+
+## Service Worker
+
+The service worker does not work with lambda functions out of the box. It prevents calling the function and returns the app itself instead ([Read more](https://github.com/facebook/create-react-app/issues/2237#issuecomment-302693219)). To solve this you have to eject and enhance the service worker configuration in the webpack config. Whitelist the path of your lambda function and you are good to go.