Update CONTRIBUTING.md

CONTRIBUTING.md

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-# Design Tokens Community Group
+# Design Tokens W3C Community Group
 
 This repository is being used for work in the W3C Design Tokens Community Group, governed by the [W3C Community License Agreement (CLA)](https://www.w3.org/community/about/agreements/cla/). To make substantive contributions, you must join the Community Group.
 
+For more details, read the [Contribution Mechanics section](https://github.com/design-tokens/community-group/blob/master/CHARTER.md#contrib) of the working group’s charter.
+
+---
+
 If you are not the sole contributor to a contribution (pull request), please identify all
 contributors in the pull request comment.