Create CONTRIBUTING.md

Added a CONTRIBUTING.md file based on the HTML Working group's [version](https://github.com/w3c/encrypted-media/blob/gh-pages/CONTRIBUTING.md)

CONTRIBUTING.md

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+# Verifiable Claims Task Force
+
+Contributions to this repository are intended to become part of Recommendation-track documents 
+governed by the [W3C Patent Policy](https://www.w3.org/Consortium/Patent-Policy-20040205/) and
+[Document License](https://www.w3.org/Consortium/Legal/copyright-documents). To contribute, you must 
+either participate in the relevant W3C Working Group or make a non-member patent licensing
+ commitment.
+
+If you are not the sole contributor to a contribution (pull request), please identify all 
+contributors in the pull request's body or in subsequent comments.
+
+ To add a contributor (other than yourself, that's automatic), mark them one per line as follows:
+
+ ```
+ +@github_username
+ ```
+
+ If you added a contributor by mistake, you can remove them in a comment with:
+
+ ```
+ -@github_username
+ ```
+
+ If you are making a pull request on behalf of someone else but you had no part in designing the 
+ feature, you can remove yourself with the above syntax.