[css-easing-2] Document addition

Author: jakearchibald

css-easing-2/Overview.bs

@@ -779,7 +779,7 @@ Changes {#changes}
 <h3 id="changes-L1">
 Additions Since Level 1</h3>
 
-TBD
+- Added ''linear()'' function.
 
 Acknowledgements {#acknowledgements}
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