message: add section on the no MIR error
diff: @@ -105,6 +105,13 @@ You may be running `cargo miri` with a different compiler version than the one
  used to build the custom libstd that Miri uses, and Miri failed to detect that.
  Try deleting `~/.cache/miri`.
  +#### "no mir for `std::rt::lang_start_internal`"
  +
  +This means the sysroot you are using was not compiled with Miri in mind. This
  +should never happen when you use `cargo miri` because that takes care of setting
  +up the sysroot. If you are using `miri` (the Miri driver) directly, see
  +[below][testing-miri] for how to set up the sysroot.
  +
  ## Development and Debugging
  If you want to hack on miri yourself, great! Here are some resources you might
  