Update CONTRIBUTION.md

Clarify which `slang.sln` file needs to be used for cmake workflow.

Author: jkwak-work

CONTRIBUTION.md

@@ -77,11 +77,12 @@ Download and install CMake from [CMake.org/download](https://cmake.org/download)
 
 Run CMake with the following command to generate a Visual Studio 2022 Solution:
 ```
-# For VisualStudio 2022
-C:\git\slang> cmake.exe --preset vs2022
+C:\git\slang> cmake.exe --preset vs2022 # For VisualStudio 2022
+C:\git\slang> cmake.exe --preset vs2019 # For VisualStudio 2019
 ```
 
-Open slang.sln with VisualStudio IDE and build it for "x64".
+Open `build/slang.sln` with VisualStudio IDE and build it for "x64".
+> Warning: there is another file, `slang.sln`, at the root directory. When you use cmake workflow, you must use one in the `build` directory.
 
 Or you can build with a following command:
 ```