Add a localhost-only log message pointing folks to the PWA docs. (#3455)

packages/react-scripts/template/src/registerServiceWorker.js

@@ -35,6 +35,15 @@ export default function register() {
       if (isLocalhost) {
         // This is running on localhost. Lets check if a service worker still exists or not.
         checkValidServiceWorker(swUrl);
+
+        // Add some additional logging to localhost, pointing developers to the
+        // service worker/PWA documentation.
+        navigator.serviceWorker.ready.then(() => {
+          console.log(
+            'This web app is being served cache-first by a service ' +
+              'worker. To learn more, visit https://goo.gl/SC7cgQ'
+          );
+        });
       } else {
         // Is not local host. Just register service worker
         registerValidSW(swUrl);