message: make a note about CORS headers requirements
diff: @@ -73,6 +73,10 @@ if (blockstack.isUserSignedIn()) {
  }
  ```
  +Make sure your manifest.json file has appropriate CORS headers so that it can
  +be fetched via an http `GET` from any origin.
  +
  +
  7) Serve your application
  ## Operations
  