Update changelog

Author: cpsievert

CHANGELOG.md

@@ -9,6 +9,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 (mimicking the `window.location` API). `href` can be used to handle
 `pathname`, `hash`, and `search` in aggregate, or each can be manipulated
 independently.
+- The `children` property of `dash_core_components.Markdown` and 
+`dash_core_components.SyntaxHighlighter` now accepts an 
+array of strings (previously it *had* to be a string). Now,
+if an array is provided, it is collapsed into a string with line 
+breaks (see #134).
 
 ## [0.15.3] - 2017-12-11
 ### Fixed