Update CHANGELOG.md (dart-archive/yaml_edit#37)

* Fix for selecting the correct index in insertion

* Add new test cases

* Add more tests and format corrections

* Update `CHANGELOG.md`

Author: Ishad-M-I-M

pkgs/yaml_edit/CHANGELOG.md

@@ -1,3 +1,8 @@
+## v2.0.4
+- Fix YamlEditor `appendToList` and `insertIntoList` functions inserts new item into next yaml item
+  rather than at end of list.
+  ([#23](https://github.com/dart-lang/yaml_edit/issues/23))
+
 ## v2.0.3
 - Updated the value of the pubspec `repository` field.