Updated history

HISTORY.md

@@ -4,6 +4,14 @@ http://visjs.org
 
 ## not yet released, version 4.0.0-SNAPSHOT
 
+### General
+
+- Changed the build scripts to include a transpilation of ES6 to ES5
+  (using http://6to5.org), so we can use ES6 features in the vis.js code.
+  When creating a custom bundle using browserify, one now needs to add a
+  transform step using `6to5ify`, this is described in README.md.
+
+
 ### Timeline
 
 - Fixed range items not being displayed smaller than 10 pixels (twice the