[css-timing-1] Add changes section and previous version links

Author: birtles

css-timing-1/Overview.bs

@@ -7,6 +7,8 @@ Level: 1
 Group: csswg
 ED: https://drafts.csswg.org/css-timing/
 TR: https://www.w3.org/TR/css-timing-1/
+Previous version: https://www.w3.org/TR/2018/WD-css-timing-1-20180926/
+Previous version: https://www.w3.org/TR/2017/WD-css-timing-1-20170221/
 Editor: Brian Birtles, Mozilla https://www.mozilla.org/, bbirtles@mozilla.com, w3cid 43194
 Editor: Dean Jackson, Apple Inc https://www.apple.com/, dino@apple.com, w3cid 42080
 Editor: Matt Rakow, Microsoft, w3cid 62267
@@ -414,6 +416,17 @@ defined in [[CSSOM]] with the following additional requirements:
     2.   Otherwise, serialize as <a lt="steps()"
          function>steps(&lt;integer&gt;, &lt;step-position&gt;)</a>.
 
+Changes since last publication {#changes}
+==============================
+
+The following changes have been made since the <a
+  href="https://www.w3.org/TR/2017/WD-css-timing-1-20170221/">21 February
+2017 Working Draft</a>:
+
+*   Removed `frames()` timing function.
+*   Added ''steps()/jump-start'', ''steps()/jump-end'',
+    ''steps()/jump-none'', and ''steps()/jump-both'' keywords to
+    ''steps()'' timing function.
 
 Acknowledgements {#acknowledgements}
 ================