Adding baseline CONTRIBUTING.md

Author: cwilso

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# 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.