Merge pull request #452 from Microsoft/addBreakingChangeLink

Add breaking change link to release-notes

Author: mhegazy

pages/release notes/TypeScript 2.1.md

@@ -452,6 +452,8 @@ var Derived = (function (_super) {
 }(Base));
 ```
 
+> This change entails a break in the behavior of extending built-in classes like `Error`, `Array`, `Map`, etc.. Please see the [extending built-ins breaking change documnetation](https://github.com/Microsoft/TypeScript-wiki/blob/master/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work) for more details.
+
 # Configuration inheritance
 
 Often a project has multiple output targets, e.g. `ES5` and `ES2015`, debug and production or `CommonJS` and `System`;