Add contributing file

Based off of https://github.com/w3c/csvw/blob/gh-pages/contributing.md

Author: dijonkitchen

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+Contributions to this repository are intended to become part of Recommendation-track documents governed by the
+[W3C Patent Policy](http://www.w3.org/Consortium/Patent-Policy-20040205/) and
+[Document License](http://www.w3.org/Consortium/Legal/copyright-documents). 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.