Update changelog for release candidate (AcademySoftwareFoundation#2250)

Author: jstone-lucasfilm

CHANGELOG.md

@@ -1,6 +1,23 @@
 # Change Log
 
-## [1.39.3] - Development
+## [1.39.3] - Release Candidate
+
+### Added
+- Added support for the [fract node](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2169), including both new specification language and data library implementations.
+- Added support for [OSL 1.14](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2204) in shader generation, leveraging the latest closures when available.
+- Added support for [MDL 1.10](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2214) in shader generation, leveraging the latest closures when available.
+- Added initial support for [data library builds](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2226), allowing future releases to generate derived artifacts from the MaterialX data libraries.
+- Added an [ElementVec type alias](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2231) in MaterialX C++, allowing certain patterns to be expressed more clearly and concisely. 
+
+### Changed
+- Refactored the [type system](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2176) in shader generation, improving the thread-safety and robustness of the generation process.
+- Improved the implementations of the [BSDF operator nodes](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2191), preferring data definitions over explicit C++ code.
+- Improved the implementation of the [time node](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2192), using a node graph implementation in hardware shading languages.
+
+### Fixed
+- Added validation of [tree depth](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2232) and [XInclude depth](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2233) in XML parsing, addressing cases where unusually complex documents could trigger a stack overflow.
+- Adding missing [null](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2228) [checks](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2229) in the getShaderNodes helper function.
+- Improved the handling of [invalid materials](https://github.com/AcademySoftwareFoundation/MaterialX/pull/2230) in the MaterialX Viewer, addressing cases where missing lights could trigger a crash.
 
 ## [1.39.2] - 2025-01-20