Update changelog for v1.2.0

Author: mdtusz

CHANGELOG.md

@@ -3,6 +3,23 @@
 For more context information, please read through the
 [release notes](https://github.com/plotly/plotly.js/releases).
 
+## [1.2.0] -- 2015-12-07
+
+### Added
+- All `Plotly.____` methods now return a promise, and pass the graph-div as an argument [#77]
+- `package.json` now has `webpack` field so plotly.js can be used by webpack users [#68]
+- Add support for rgba colours in pie-charts [#63]
+
+### Changed
+- Optimized by calling window.getComputedStyle only once [#81]
+- Improved npm install process for topojson file resolve [#48]
+
+### Fixed
+- Fixed geo-plot promises [#52]
+- Typo fixed in `No DOM element` error message [#64]
+- Fix hover labels with hoverinfo 'text' [#70]
+- Fix scaling for gl3d error bars [#74]
+
 ## [1.1.1] -- 2015-12-01
 
 ### Fixed