Update contribution file with WebAppSec

DBSC has been adopted by WebAppSec so remove the reference to the CG in
CONTRIBUTING.md.

Author: Daniel Rubery

CONTRIBUTING.md

@@ -1,8 +1,9 @@
 # Device Bound Session Credentials
 
-This repository is being used for work in the W3C Device Bound Session Credentials, governed by the [W3C Community License
-Agreement (CLA)](http://www.w3.org/community/about/agreements/cla/). To make substantive contributions,
-you must join the CG.
+This repository is being used for work in the W3C Device Bound Session
+Credentials, governed by the [W3C Community License Agreement
+(CLA)](http://www.w3.org/community/about/agreements/cla/). It is being developed
+in the WebAppSec WG. To make substantive contributions, you must join the WG.
 
 If you are not the sole contributor to a contribution (pull request), please identify all
 contributors in the pull request comment.