[skip-ci][doc] Add TDirectoryFile::mkdir changes to release notes

Author: silverweed

README/ReleaseNotes/v640/index.md

@@ -44,6 +44,8 @@ The following people have contributed to this new version:
 ## Core Libraries
 
 ## I/O
+* The behavior or `TDirectoryFile::mkdir` (which is also `TFile::mkdir`) was changed regarding the creation of directory hierarchies: calling `mkdir("a/b/c", "myTitle")` will now assign `myTitle` to the innermost directory `"c"` (before this change it would assign it to `"a"`).
+* Fixed a bug in `TDirectoryFile::mkdir` where passing `returnExistingDirectory = true` would not work properly in case of directory hierarchies. The option is now correctly propagated to `mkdir`'s inner invocations.
 
 ### TTree