readme: add 2.1.0 release to mini changelog

Author: paulirish

README.md

@@ -163,7 +163,9 @@ Copyright Jason McCreary & Paul Irish. Licensed under MIT.
 
 ## Changelog
 
-- **2017-12-01** - 2.0 shipped. Breaking change: [Gitlab configuration](https://github.com/paulirish/git-open#configuration) handled differently.
+
+- **2018-12-03** - [2.1.0 shipped](https://github.com/paulirish/git-open/releases/tag/v2.1.0). 
+- **2017-12-01** - [2.0 shipped](https://github.com/paulirish/git-open/releases/tag/v2.0.0). Breaking change: [Gitlab configuration](https://github.com/paulirish/git-open#configuration) handled differently.
 - **2017-12-01** - Configuration for custom remote added
 - **2017-11-30** - Support for VSTS Added
 - **2017-10-31** - `--issue` and `-h` added