Update build steps for MacOS

This commit adds a quick build step for MacOS on CONTRIBUTION.md.

It is unclear what are the conditions that slang-llvm needs to be
rebuilt locally. An issue is filed for it, shader-slang#4812

CONTRIBUTION.md

@@ -105,6 +105,42 @@ Build with a following command:
 $ cmake --build --preset release
 ```
 
+#### MacOS
+Install XCode from AppStore.
+
+Install CMake and Ninja; we recommend using [homebrew](https://brew.sh/) for installing them.
+```
+brew install ninja
+brew install cmake
+```
+
+Run CMake with a following command to generate Makefile:
+```
+$ cmake --preset default
+```
+
+Build with a following command:
+```
+$ cmake --build --preset release
+```
+
+#### MacOS with slang-llvm local build
+slang-llvm is required to run slang-test properly.
+Depending on your hardware, your slang-llvm may need to be locally rebuilt with the following command.
+```
+external/build-llvm.sh --source-dir=build/slang-llvm_src --install-prefix=build/slang-llvm_install
+```
+
+You need to use the following command to re-generate Makefile,
+```
+$ cmake --preset default --fresh -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM -DLLVM_DIR=build/slang-llvm_install/lib/cmake/llvm -DClang_DIR=build/slang-llvm_install/lib/cmake/clang
+```
+
+Build with a following command:
+```
+$ cmake --build --preset release
+```
+
 ### Making Changes
 Make your changes and ensure to follow our [Design Decisions](docs/design/README.md).