update changelog

Author: hertg

CHANGELOG.md

@@ -1,3 +1,4 @@
 ## :lady_beetle: Fixes
 
-- Updated PKGBUILD to enclose `${srcdir}` in quotes
+- Fix bug where sometimes no default session was selected ([#18](https://github.com/hertg/lightdm-neon/issues/18) via [#20](https://github.com/hertg/lightdm-neon/pull/20))
+- Prevent overflow in background image selection (via [#22](https://github.com/hertg/lightdm-neon/pull/22), thanks @Rufi512)