Update README.md

Author: Botium

README.md

@@ -83,6 +83,9 @@ The secret for accessing your chatbot from the Azure Portal.
 
 _Note: it is advised to not add this secret to a configuration file but hand it over by environment variable BOTIUM_DIRECTLINE3_SECRET_
 
+### DIRECTLINE3_DOMAIN
+If not using the default Direct Line endpoint, e.g. if you are using a region-specific endpoint, put its full URL here
+
 ### DIRECTLINE3_WEBSOCKET
 _true or false_
 Wether to use Websocket connection or HTTP polling. Usually, using Websockets is prefered, but sometimes this is not possible (maybe with blocking proxies).
@@ -104,4 +107,4 @@ _Note: if you want to disable this, then set DIRECTLINE3_BUTTON_TYPE to "message
 
 ## Open Issues and Restrictions
 
-* Media Attachments currently cannot be sent, as Node.js environment is [not fully supported by DirectLine](https://github.com/Microsoft/BotFramework-DirectLineJS/issues/107)
+* Media Attachments currently cannot be sent, as Node.js environment is [not fully supported by DirectLine](https://github.com/Microsoft/BotFramework-DirectLineJS/issues/107)