message: Mention dirs crate
diff: //! rustup, as well functions to find the correct value of
  //! `CARGO_HOME` and `RUSTUP_HOME`.
  //!
  +//! See also the [`dirs`](https://docs.rs/dirs) crate.
  +//!
  +//! _Note that as of 2019/08/06 it appears that cargo uses this crate, but
  +//! rustup does not._
  +//!
  //! The definition of `home_dir` provided by the standard library is
  //! incorrect because it considers the `HOME` environment variable on
  //! Windows. This causes surprising situations where a Rust program
  