[flang] Merge pull request flang-compiler/f18#1087 from CarolineConcatto/update_readme

Adjust supported compilers statement in F18 readme

Original-commit: flang-compiler/f18@6e04d35b895dea17d432f2b8137fabee1c625ed3
Reviewed-on: https://github.com/flang-compiler/f18/pull/1087
tree: 6768f59f0f7eb23425805aea7cd5604e2da4c855
  1. flang/