Update Whats_New.md for Metal3 argument buffers.

Docs/Whats_New.md

@@ -18,10 +18,19 @@ MoltenVK 1.2.10
 
 Released TBD
 
+- Improvements to bindless resources and descriptor indexing:
+ - Add support for Metal3 argument buffers.
+ - Support argument buffers on all platforms, when Metal 3 is available.
+ - Support argument buffers on macOS when Metal3 is not avaiable.
+ - Use Metal argument buffers by default when they are available.
+ - Revert MVKConfiguration::useMetalArgumentBuffers and env var 
+ `MVK_CONFIG_USE_METAL_ARGUMENT_BUFFERS` to boolean value, and enable it by default.
+ - Update max number of bindless buffers and textures per stage to 1M, per Apple Docs.
 - Add option to generate a GPU capture via a temporary named pipe from an external process.
 - Fix shader conversion failure when using native texture atomics.
 - MSL shader conversion, only pass resource bindings that apply to current shader stage.
 - Update documentation for minimum runtime OS requirements to indicate _macOS 10.15_, _iOS 13_, or _tvOS 13_.
+- Update `MVK_PRIVATE_API_VERSION` to version `43`.
 - Update to latest SPIRV-Cross:
  - MSL: Add option to force depth write in fragment shaders
  - MSL: Improve handling of padded descriptors with argument buffers