updated readme with supported compilers

Author: Hana Dusíková

README.md

@@ -119,6 +119,7 @@ Otherwise you will get missing symbols if you try to use the unicode support wit
 
 * clang 6.0+ (template UDL, C++17 syntax)
 * xcode clang 10.0+ (template UDL, C++17 syntax)
+* clang 12.0+ (C++17 syntax, C++20 cNTTP syntax)
 * gcc 8.0+ (template UDL, C++17 syntax)
 * gcc 9.0+ (C++17 & C++20 cNTTP syntax)
 * MSVC 15.8.8+ (C++17 syntax only) (semi-supported, I don't have windows machine)