message: explain our flags in the README
diff: @@ -55,6 +55,14 @@ miri` to run your project, if it is a bin project, or run
  `MIRI_SYSROOT=~/.xargo/HOST cargo +nightly miri test` to run all tests in your
  project through Miri.
  +## Miri `-Z` flags
  +
  +Miri adds some extra `-Z` flags to control its behavior:
  +
  +* `-Zmiri-start-fn`: This makes interpretation start with `lang_start` (defined
  + in libstd) instead of starting with `main`. Requires full MIR!
  +* `-Zmiri-disable-validation` disables enforcing the validity invariant.
  +
  ## Development and Debugging
  Since the heart of Miri (the main interpreter engine) lives in rustc, working on
  