[web-animations-2] Link to issue

Author: fantasai

web-animations-2/Overview.bs

@@ -155,6 +155,7 @@ previous level of this specification:
 This module also includes some exploratory proposals for [[#custom-effects]],
 however there are some concerns about the design of this feature
 and it may be replaced in a future revision.
+See <a href="https://github.com/w3c/csswg-drafts/issues/6861">discussion</a>.
 </div>
 
 <h2 id="timing-model">Timing model</h2>