add link to PR

Author: dfreedm

CHANGELOG.md

@@ -20,7 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 * Properties annotated with the `eventOptions` decorator will now survive property renaming optimizations when used with tsickle and Closure JS Compiler.
-* Moved style gathering from `finalize` to `initialize` to be more lazy, and create stylesheets on the first instance initializing.
+* Moved style gathering from `finalize` to `initialize` to be more lazy, and create stylesheets on the first instance initializing [#866](https://github.com/Polymer/lit-element/pull/866).
 
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