Release Note for OS-normalized include paths

The behavior was changed with #6317 so that regardless of spelling in
the shader, the include path will conform to the host OS style for the
purposes of the include handler. This just adds a release note for that
new behavior.

Fixes #6669

docs/ReleaseNotes.md

@@ -69,6 +69,9 @@ DX Compiler release for March 2024
   - Validation errors more accurately determine usage by the entry point
 - Improve debug info generation
 - Further improvements to Linux build quality
+- Include paths input to custom include handlers are now normalized to the OS path style
+  - This means that regardless of how they are denoted in the shader, the string passed to `IDxcIncludeHandler::LoadSource` will use OS-normalized paths
+  - This could break existing code
 
 ### Version 1.7.2308