Updated changelog

Author: vkbo

CHANGELOG.md

@@ -7,6 +7,10 @@
 * Fixed a critical GUI bug when trying to create new folders and files in the tree.
 * Caught a bug when creating a new file, but novelWriter couldn't figure out what class the parent item had and returned a None. Could not recreate the bug, but added a check for it.
 
+**Code Improvements**
+
+* Changed the way user alerts are generated, and added the alert levels to an enum class named `nwAlert`. Also added a new level named `BUG`.
+
 ## Version 0.1.1 [2019-05-12]
 
 **User Interface**