[flang] Update README.md with a list of compilers that are known to

compile f18. Explain how to configure LLVM on the cmake cmd
line. Add more detail about how to compile with clang++.

Original-commit: flang-compiler/f18@e7f678147bc842188d90beafc77e2c84c614239d
Reviewed-on: https://github.com/flang-compiler/f18/pull/344
Tree-same-pre-rewrite: false