Add issue about player sequence number being based on creation time, not attachment time

Author: birtles

web-animations/index.html

@@ -586,6 +586,10 @@ <h2>Players</h2>
         of situations such as combining animations, queuing events, and
         returning the list of current players.
       </p>
+      <p class="issue">
+        Should this actually be based on when the player is attached to
+        a timeline?
+      </p>
       <section>
         <h3>The current time of a player</h3>
         <p>