message: expand README
diff: @@ -286,6 +286,8 @@ different Miri binaries, and as such worth documenting:
  directory after loading all the source files, but before commencing
  interpretation. This is useful if the interpreted program wants a different
  working directory at run-time than at build-time.
  +* `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
  + perform verbose logging.
  [testing-miri]: CONTRIBUTING.md#testing-the-miri-driver
  