update changelog for 1.21.2

Author: etpinard

CHANGELOG.md

@@ -9,6 +9,14 @@ https://github.com/plotly/plotly.js/compare/vX.Y.Z...master
 
 where X.Y.Z is the semver of most recent plotly.js release.
 
+
+## [1.21.2] -- 2016-12-14
+- Fix handling of calendar in `filter` transforms where distinct calendars can
+  now be set for both the `target` and `value` [#1253]
+- Make `Plotly.addFrames` skip over non-plain-objects inputs [#1254]
+- Make `Plots.graphJson` aware of `frames` [#1255]
+
+
 ## [1.21.1] -- 2016-12-14
 
 ### Fixed