docs: add info about HTTP caching headers into Firebase section (facebook#3659)

Author: bobrosoft

template/README.md

@@ -2086,6 +2086,18 @@ Then run the `firebase init` command from your project’s root. You need to cho
     ✔  Firebase initialization complete!
 ```
 
+IMPORTANT: you need to set proper HTTP caching headers for `service-worker.js` file in `firebase.json` file or you will not be able to see changes after first deployment ([issue #2440](https://github.com/facebookincubator/create-react-app/issues/2440)). It should be added inside `"hosting"` key like next:
+
+```
+{
+  "hosting": {
+    ...
+    "headers": [
+      {"source": "/service-worker.js", "headers": [{"key": "Cache-Control", "value": "no-cache"}]}
+    ]
+    ...
+```
+
 Now, after you create a production build with `npm run build`, you can deploy it by running `firebase deploy`.
 
 ```sh