Adds instructions for adding admin users.

docs/Running-Mastodon/Heroku-guide.md

@@ -11,3 +11,5 @@ Mastodon can theoretically run indefinitely on a free [Heroku](https://heroku.co
   * You will want Amazon S3 for file storage. The only exception is for development purposes, where you may not care if files are not saved. Follow a guide online for creating a free Amazon S3 bucket and Access Key, then enter the details.
   * If you want your Mastodon to be able to send emails, configure SMTP settings here (or later). Consider using [Mailgun](https://mailgun.com) or similar, who offer free plans that should suit your interests.
 3. Deploy! The app should be set up, with a working web interface and database. You can change settings and manage versions from the Heroku dashboard.
+
+You may need to use the `heroku` CLI application to modify the database directly to give yourself administration rights.