Add warning about CMake version on CONTRIBUTION.md

Currently CMake version is required to be 3.20 or above.
The version requirement is properly defined in our CMakeList.txt file.
But older versions of CMake may not even print an error about the version requirement.

Author: jkwak-work

CONTRIBUTION.md

@@ -94,6 +94,7 @@ Install CMake and Ninja.
 ```
 $ sudo apt-get install cmake ninja-build
 ```
+> Warning: Currently the required CMake version is 3.20 or above.
 
 Run CMake with a following command to generate Makefile:
 ```