Add the extra step to install extension manually (#12299)

ee/fleetd-chrome/README.md

@@ -19,15 +19,30 @@ To learn how to package and add hosts to Fleet, visit: https://fleetdm.com/docs/
 
 View service worker logs in chrome://serviceworker-internals/?devtools (in production), or in chrome://extensions (only during development).
 
-### Dev
+### Manual Enroll
+
+> Steps 1 and 2 can be performed on your workstation. Step 3 and 4 are to be executed on the target Chromebook.
 
 1. Create your .env file:
-```
-echo 'FLEET_URL="<some_url>"' >> .env
+
+> IMPORTANT: The address in `FLEET_URL` must have a valid TLS certificate.
+
+```sh
+echo 'FLEET_URL="https://your-fleet-server.example.com"' >> .env
 echo 'FLEET_ENROLL_SECRET="<your enroll secret>"' >> .env
 ```
-2. Build:
+
+To test with your local Fleet server, you can use ngrok:
+```sh
+ngrok http https://localhost:8080
 ```
+
+2. Build the "unpacked extension":
+```sh
 npm install && npm run build
 ```
-3. The unpacked extension is in the `dist` dir.
+The above command will generate an unpacked extension in `./dist`.
+
+3. Send the `./dist` folder to the target Chromebook.
+
+4. In the target Chromebook, go to `chrome://settings`, toggle `Developer mode` and click on `Load unpacked` and select the `dist` folder.