update changelog for 1.21.1

Author: etpinard

CHANGELOG.md

@@ -9,6 +9,12 @@ https://github.com/plotly/plotly.js/compare/vX.Y.Z...master
 
 where X.Y.Z is the semver of most recent plotly.js release.
 
+## [1.21.1] -- 2016-12-14
+
+### Fixed
+- Fix `ms2datetime` routine for Chinese calendar [#1252]
+- Fix `tickformat` for world calendars [#1252]
+
 
 ## [1.21.0] -- 2016-12-12