Added note about POST Content-Type of text/plain

Author: Jason Moon

README.md

@@ -4,10 +4,11 @@ Implements automatic *Cross Origin Resource Sharing* support using the `XDomainR
 > **CORS** requires the `Access-Control-Allow-Origin` header to be present in the AJAX response from the server.
 
 In order to use XDomainRequest in Internet Explorer, the request must be:
-- asynchronous
-- GET or POST
-- HTTP or HTTPS
-- same scheme as calling page
+- Only GET or POST
+ - When POSTing, the data will always be sent with a `Content-Type` of `text/plain`
+- Only HTTP or HTTPS
+ - Protocol must be the same scheme as the calling page
+- Always asynchronous
 
 Working example here:
 http://jsfiddle.net/MoonScript/Q7bVG/