message: Mention +nightly in ARCHITECTURE.md
diff: @@ -104,7 +104,9 @@ assert_that(
  ```
  Alternatively to build and run a custom version of cargo simply run `cargo build`
  -and execute `target/debug/cargo`.
  +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 directory.
  Because the test suite has `#![deny(warnings)]` at times you might find it
  convenient to override this with `RUSTFLAGS`, for example
  