add token environment variable docs

Author: wj-Mcat

README.md

@@ -46,6 +46,14 @@ Here's a great live coding video tutorial from our Python Wechaty creator @wj-Mc
     export WECHATY_PUPPET=wechaty-puppet-service
     export WECHATY_PUPPET_SERVICE_TOKEN=your_token_at_here
     ```
+   
+    or you can use `TOKEN` or `token` environment variable alias name to set **token**, for example:
+
+    ```shell
+    export TOKEN=your_token_at_here
+    # or
+    export token=your_token_at_here
+    ```
 
 4. Run the bot