Update license files to match latest model

CONTRIBUTING.md

@@ -1,8 +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 bring substantive contributions to specifications, you must either participate
+[W3C Patent Policy](https://www.w3.org/Consortium/Patent-Policy/) and
+[Software and Document License](https://www.w3.org/Consortium/Legal/copyright-software). 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 
-commit message. In the commit, please include on a new line,
-<pre>Contributors: +@githubusername1, +@githubusername2, ...</pre>
+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

@@ -1,2 +1,4 @@
-All documents in this Repository are licensed by contributors under the [W3C Document
-License](http://www.w3.org/Consortium/Legal/copyright-documents).
+All documents in this Repository are licensed by contributors
+under the 
+[W3C Software and Document License](https://www.w3.org/Consortium/Legal/copyright-software).
+