Ask for tests for normative changes in CONTRIBUTING.md (#32)

See https://github.com/foolip/testing-in-standards/blob/master/policy.md
for context. This phrasing matches that used for many other specs'
CONTRIBUTING.md files.

Author: foolip

CONTRIBUTING.md

@@ -22,3 +22,12 @@ If you added a contributor by mistake, you can remove them in a comment with:
 
 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.