update changelog

Author: Stephan Dilly

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - introduced proper changelog
 
 ### Changed
+- better error message when trying to run outside of a valid git repo ([#56](https://github.com/extrawurst/gitui/issues/56))
 - improve ctrl+c handling so it is checked first and no component needs to worry of blocking it
 
 ## [0.2.3] - 2020-05-12