[doc] Update changelog for 4.0.1

Author: Thoemi09

doc/ChangeLog.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## Version 4.0.1
+
+omegamaxent_interface version 4.0.1 is a patch-release that introduces
+minor documentation updates.
+
+We thank all contributors: Thomas Hahn, Alexander Hampel
+
+Find below an itemized list of changes in this release.
+
+### doc
+* Add a version switcher flyout to the sphinx docs
+* Add canonical URL pointing to latest/ for SEO
+
+
 ## Version 4.0.0
 
 omegamaxent_interface version 4.0.0 is a compatibility release for TRIQS version 4.0.0 that