Update README.md (#4465)

Delete 2016 references.  Add link to WAI GitHub Onboarding wiki.

Co-authored-by: Patrick H. Lauke <[email protected]>
Co-authored-by: Mike Gower <[email protected]>

Author: 3 people

README.md

@@ -1,10 +1,10 @@
-WCAG (Web Content Accessibility Guidelines) - Branch for production of August 2016 review versions
+WCAG (Web Content Accessibility Guidelines) 2
 ===
 
-Please use this branch as the target for pull requests until July 10, 2016.
-
 This repository is used to develop content for WCAG 2, as well as associated understanding documents and techniques.
 
+See [Onboarding for working in Github](https://www.w3.org/WAI/GL/wiki/Onboarding_for_working_in_Github)
+
 ## Editorial style guide
 
 @@To complete