message: added line indicating that `build.sh` and `cargo miri` need the same toolchain
diff: @@ -71,6 +71,10 @@ with the date replaced as appropriate.
  2. Run `xargo/build.sh` as `rustup run nightly-2018-10-15 build.sh`.
  3. When running tests, use `MIRI_SYSROOT=~/.xargo/HOST cargo +nightly-2018-10-15 miri test`.
  +You may prefer to do this rather than depending on the rustup default toolchain,
  +if you routinely update the default, since **it is essential that `xargo/build.sh`
  +is run with the same toolchain as `cargo miri`.**
  +
  ## Miri `-Z` flags
  Miri adds some extra `-Z` flags to control its behavior:
  