README: update build instructions to include c++20 compilers

Hopefully, this helps to prevent people to run into issues like this:
zxing-cpp#834 (comment)

README.md

@@ -98,7 +98,7 @@ As an example, have a look at [`ZXingWriter.cpp`](example/ZXingWriter.cpp).
 These are the generic instructions to build the library on Windows/macOS/Linux. For details on how to build the individual wrappers, follow the links above.
 
 1. Make sure [CMake](https://cmake.org) version 3.15 or newer is installed.
-2. Make sure a C++17 compliant compiler is installed (minimum VS 2019 16.8 / gcc 7 / clang 5).
+2. Make sure a sufficiently C++20 compliant compiler is installed (minimum VS 2019 16.10? / gcc 11 / clang 12?).
 3. See the cmake `ZXING_...` options to enable the testing code, python wrapper, etc.
 
 ```