update changelog for v4.0.3

Author: jszuminski

CHANGELOG.md

@@ -4,6 +4,16 @@ _New Features:_
 
 - Added support for absolute paths in the `HIGHCHARTS_CACHE_PATH` option [(#562)](https://github.com/highcharts/node-export-server/issues/562)
 
+_Fixes_:
+
+- Improved status codes (user errors instead of 500) [(#577)](https://github.com/highcharts/node-export-server/pull/577)
+
+- Improved memory management/usage [(#586)](https://github.com/highcharts/node-export-server/pull/586)
+
+_Other:_
+
+- Add fair usage policy note on the page [(#583)](https://github.com/highcharts/node-export-server/pull/583)
+
 # 4.0.2
 
 _Hotfix_: