[css-flexbox] Add Animatable line changes to changes list.

Author: fantasai

css-flexbox/Overview.bs

@@ -3505,6 +3505,8 @@ Clarifications</h4>
 			</blockquote>
 		<li id="change-2016-clarify-staticpos">
 			Slightly reworded the section on determining the static position of absolutely-positioned children to be clearer.
+		<li id="change-2016-animation-type">
+			Adjusted format of Animatable lines to be clearer about animating keywords.
 		<li>Miscellaneous trivial editorial improvements.
 	</ul>