[css-contain] Mention the test suite in the changelog

Author: frivoal

css-contain-1/Overview.bs

@@ -553,6 +553,7 @@ This appendix is <em>informative</em>.
 * Change the animation type of the contain property from discrete to not animatable
 * Define the effect of containment on SVG elements
 * Editorial improvements
+* A comprehensive test suite for the full specification was developped, see <a href="http://test.csswg.org/harness/review/css-contain-1_dev">http://test.csswg.org/harness/review/css-contain-1_dev</a>
 
 <h3 id="2017-08-08-changes">Changes from the
 <a href="https://www.w3.org/TR/2017/CR-css-contain-1-20170808/">Candidate Recommendation of 8 August 2017</a></h3>