message: Finalize changelog for v1.38.0
diff: # Change Log
  -## [1.38.0] - Development
  +## [1.38.0] - 2021-03-02
  Updated the MaterialX library to the v1.38 specification. See the [v1.38 changelist](http://www.materialx.org/assets/MaterialX.v1.38.Changelist.pdf) for full details.
  ### Added
  - Added support for the generalized 'layer' node in Physically Based Shading.
  +- Added user controls for texture baking and wedge rendering in the [MaterialX Viewer](https://github.com/materialx/MaterialX/blob/main/documents/DeveloperGuide/Viewer.md).
  - Added support for Nvidia's Material Definition Language (MDL) in MaterialX code generation.
  - Added support for inline source code in Implementation elements.
  - Added support for TargetDef elements.
  -- Added user controls for texture baking and wedge rendering in the MaterialX viewer.
  - Added viewer rendering to cloud-based tests in GitHub Actions.
  - Added support for Xcode 12.
  