Added CONTRIBUTING and LICENSE for WICG.

Author: mgiuca

CONTRIBUTING.md

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+# Web Platform Incubator Community Group
+
+This repository is being used for work in the W3C Web Platform Incubator
+Community Group, 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.md

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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/).