Adding pont asking to enable billing in the GitHub to Slack example.

See #24

github-to-slack/README.md

@@ -21,6 +21,7 @@ The dependencies are listed in [functions/package.json](functions/package.json).
 To test this sample action:
 
  - Create a Firebase Project using the [Firebase Developer Console](https://console.firebase.google.com)
+ - Enable billing on your project by switching to the Blaze or Flame plan. See [pricing](https://firebase.google.com/pricing/) for more details. This is required to be able to do requests to non-Google services.
  - Configure this sample to use your project using `firebase --use add` and select your project.
  - [Add a WebHook to your GitHub repo](https://help.github.com/articles/about-webhooks/) with the following settings:
    - Payload URL: `https://us-central1-<FIREBASE_PROJECT_ID>.cloudfunctions.net/githubWebhook`