Change license to conform to W3C CG license. Create contributing.md

Author: Peter Rushforth

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# @CommunityGroup@
+
+This repository is being used for work in the W3C @CommunityGroup@, governed by the [W3C Community License Agreement (CLA)](http://www.w3.org/community/about/agreements/cla/). To make substantive contributions, you must join the CG. 
+
+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.

LICENSE

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+All Reports in this Repository are licensed by Contributors
+under the 
+[W3C Software and Document License](http://www.w3.org/Consortium/Legal/2015/copyright-software-and-document).  
+Contributions to Specifications are made under the
+[W3C CLA](https://www.w3.org/community/about/agreements/cla/).