message: Add a note about m4/flex/bison to the README file
diff: @@ -45,6 +45,8 @@ Verible's code base is written in C++.
  To build, you need the [bazel] build system and a C++17 compatible compiler
  (e.g. >= g++-9), as well as python3.
  +Linux systems also require the following tools: m4, flex, bison.
  +
  Use your package manager to install the dependencies; on a system with
  the nix package manager simply run `nix-shell` to get a build environment.
  