Create CONTRIBUTING.md

Add based on that used in the [Web Platform tests](https://raw.githubusercontent.com/w3c/web-platform-tests/master/CONTRIBUTING.md).

Author: gkellogg

CONTRIBUTING.md

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+# RDF Tests Community Group
+
+## Grant of License
+
+By contributing to this repository, you and the company you represent, if the
+company holds any copyrights in the contribution, grant to the W3C a perpetual,
+non-exclusive, royalty-free, world-wide right and license under all of your
+copyrights in this contribution to copy, publish, use, and modify the
+contribution and to distribute the contribution under a BSD License or one with
+more restrictive terms, as well as a right and license of the same scope to any
+derivative works prepared by the W3C and based on or incorporating all or part
+of the contribution. You further agree that any derivative works of this
+contribution prepared by the W3C shall be solely owned by the W3C.
+
+You state, to the best of your knowledge, that you, or the company you
+represent, have all rights necessary to contribute the materials.
+
+W3C will retain attribution of initial authorship to you. The W3C makes no
+a-priori commitment to support or distribute contributions.
+
+## Disclaimer
+
+All content from this repository is provided as is, and W3C makes no
+representations or warranties, express or implied, including, but not limited
+to, warranties of merchantability, fitness for a particular purpose,
+non-infringement, or title; nor that the contents of this repository are
+suitable for any purpose. We make no representations, express or implied, that
+the content of this repository or the use thereof indicates conformance to a
+specification. All content is provided as-is to help reach interoperability.