added documentation for telegram relay bot feature

docs/configuring-playbook-bridge-mautrix-telegram.md

@@ -13,3 +13,9 @@ matrix_mautrix_telegram_api_hash: YOUR_TELEGRAM_API_HASH
 ```
 
 You then need to start a chat with `@telegrambot:YOUR_DOMAIN` (where `YOUR_DOMAIN` is your base domain, not the `matrix.` domain).
+
+If you want to use the relay-bot feature ([relay bot documentation](https://github.com/tulir/mautrix-telegram/wiki/Relay-bot)), which allows anonymous user to chat with telegram users, use the following addtional playbook configuration:
+
+```yaml
+matrix_mautrix_telegram_bot_token: YOUT_TELEGRAM_BOT_TOKEN
+```