Ask for tests for normative changes in README (w3c#493)

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, in particular:
https://github.com/w3c/fxtf-drafts/blob/master/CONTRIBUTING.md#tests
https://github.com/w3c/csswg-drafts/blob/master/CONTRIBUTING.md#tests (slightly different)

Author: foolip

README.markdown

@@ -10,3 +10,12 @@ We also maintain the [public-houdini mailing list](http://lists.w3.org/Archives/
 
 New specifications are generally first incubated in the WICG, in particular:
 - [Animation Worklet](https://github.com/WICG/animation-worklet)
+
+# 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.