Ask for web-platform-tests in CONTRIBUTING.md

The wording is adapted from the WHATWG contributor guidelines:
https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md

This is sometimes already happening:
web-platform-tests/wpt#3449
web-platform-tests/wpt#5628

Drive-by: whitespace

CONTRIBUTING.md

@@ -10,13 +10,15 @@ Updates to the spec must reference [resolved issues marked `needs spec`](https:/
 
 To make edits to the design, please send pull requests against the Nightly spec on the master branch. We use [bikeshed](https://github.com/tabatkins/bikeshed). So, change `docs/index.bs` and submit it with the corresponding bikesheded `index.html`.
 
-Contributions to this repository are intended to become part of Recommendation-track documents 
+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.
+
+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:
@@ -31,5 +33,5 @@ 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.