Create CONTRIBUTING.md

CONTRIBUTING.md

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+# W3C Credentials Community Group
+
+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
+[Software and Document License](https://www.w3.org/Consortium/Legal/copyright-software).
+To make substantive contributions to specifications, 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 comment.
+
+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.