Update release date of 2.1.0

Author: thiemowmde

README.md

@@ -21,7 +21,7 @@ version 2.0 of this package:
 
 ## Release notes
 
-### 2.1.0 (dev)
+### 2.1.0 (2017-09-04)
 
 * Updated the MediaWiki entry point to use the extension.json format.
 * Added code sniffers for JavaScript as well as PHP.