message: Detail how to run locally-built nightly cargo
  Documenting from
diff: @@ -106,7 +106,10 @@ assert_that(
  Alternatively to build and run a custom version of cargo simply run `cargo build`
  and execute `target/debug/cargo`. Note that `+nightly`/`+stable` (and variants),
  being [rustup](https://rustup.rs/) features, won't work when executing the locally
  -built cargo binary directly.
  +built cargo binary directly, you have to instead build with `cargo +nightly build`
  +and run with `rustup run` (e.g `rustup run nightly
  +<path-to-cargo>/target/debug/cargo <args>..`) (or set the `RUSTC` env var to point
  +to nightly rustc).
  Because the test suite has `#![deny(warnings)]` at times you might find it
  convenient to override this with `RUSTFLAGS`, for example
  