message: add contributing notes for cross compile test setup
diff: @@ -78,7 +78,10 @@ in `test/` provide templates on how to test Cargo's behavior in a
  sandbox-environment. The internal crate `cargotest` provides a vast amount
  of helpers to minimize boilerplate.
  * Make sure `cargo test` passes. If you do not have the cross-compilers
  -installed locally, ignore the cross-compile test failures or disable them by
  +installed locally, install them using the instructions returned by
  +`cargo test cross_compile::cross_tests` (twice, with `--toolchain nightly`
  +added to get the nightly cross target too); alternatively just
  +ignore the cross-compile test failures or disable them by
  using `CFG_DISABLE_CROSS_TESTS=1 cargo test`. Note that some tests are enabled
  only on `nightly` toolchain. If you can, test both toolchains.
  * All code changes are expected to comply with the formatting suggested by `rustfmt`.
  