Add release notes for umask file permissions change

Author: Ahmed Bilal

README/ReleaseNotes/v640/index.md

@@ -45,8 +45,25 @@ 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.
+### File Permissions Now Respect System `umask`
+
+ROOT now respects the system `umask` when creating files, following standard Unix conventions. 
+
+**Previous behavior:** Files were created with hardcoded `0644` permissions (owner read/write, group/others read-only), ignoring the system `umask`.
+
+**New behavior:** Files are created with `0666` permissions masked by the system `umask` (`0666 & ~umask`), consistent with standard Unix file creation functions like `open()` and `fopen()`.
+
+**Impact:**
+- **Users with default `umask 022`** (most common): No change - files are still created as `0644`
+- **Users with stricter `umask` values** (e.g., `0077`): Files will now be created with more restrictive permissions (e.g., `0600` - user-only access)
+- **Users with permissive `umask` values** (e.g., `0002`): Files may be created with slightly more open permissions (e.g., `0664` - group-writable)
+
+**Note:** If you require specific file permissions regardless of `umask`, you can set `umask` explicitly before running ROOT (e.g., `umask 022`) or use `chmod` after file creation.
+
+This change affects the following classes:  `TFile`, `TMapFile`, `TMemFile`, `TDCacheFile`, `TFTP`, and `TApplicationServer`.
 
 ### TTree