Ask for web-platform-tests in CONTRIBUTING.md (#200)

The wording is adapted from the following documents:
https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md#tests
https://github.com/whatwg/meta/blob/master/TEAM.md
https://github.com/w3c/ServiceWorker/blob/master/CONTRIBUTING.md
https://github.com/w3c/web-performance/blob/gh-pages/CONTRIBUTING.md#test-driven

Drive-by: whitespace

CONTRIBUTING.md

@@ -1,12 +1,12 @@
 # Web Platform Working Group
 
-Contributions to this repository are intended to become part of Recommendation-track documents 
+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 contribute, you must 
+[Document License](http://www.w3.org/Consortium/Legal/copyright-documents). To contribute, you must
 either participate in the relevant W3C Working Group or make a non-member patent licensing
- commitment.
+commitment.
 
-If you are not the sole contributor to a contribution (pull request), please identify all 
+If you are not the sole contributor to a contribution (pull request), please identify all
 contributors in the pull request's body or in subsequent comments.
 
  To add a contributor (other than yourself, that's automatic), mark them one per line as follows:
@@ -21,5 +21,14 @@ contributors in the pull request's body or in subsequent comments.
  -@github_username
  ```
 
- If you are making a pull request on behalf of someone else but you had no part in designing the 
+ 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.
+
+# Tests
+
+For normative changes, a corresponding
+[web-platform-tests](https://github.com/w3c/web-platform-tests) PR is highly appreciated. Typically,
+both PRs will be merged at the same time. Note that a test change that contradicts the spec should
+not be merged before the corresponding spec change. If testing is not practical, please explain why
+and if appropriate [file an issue](https://github.com/w3c/web-platform-tests/issues/new) to follow
+up later. Add the `type:untestable` or `type:missing-coverage` label as appropriate.