Added documentation for custom region environment variable

README.md

@@ -41,7 +41,7 @@ Additionally, you can specify one of several environment variable (via `-e`) to
   * `NGROK_PASSWORD` - Password to use for HTTP authentication on the tunnel. You must also specify an authentication token.
   * `NGROK_PROTOCOL` - Can either be `HTTP` or `TCP`, and it defaults to `HTTP` if not specified. If set to `TCP`, Ngrok will allocate a port instead of a subdomain and proxy TCP requests directly to your application.
   * `NGROK_PORT` - Port to expose (defaults to `80` for `HTTP` protocol).
-  * 
+  * `NGROK_REGION` - Location of the ngrok tunnel server; can be `us` (United States, default), `eu` (Europe), `ap` (Asia/Pacific) or `au` (Australia)
 
 #### Full example