message: Dependency policy: add a note that the dependencies discussed are for building apps, and expand the "libcore" bullet to the entire toolchain.
diff: Third Party Dependencies
  ========================
  +This document is about dependencies that are required to build applications
  +using `libtock-rs`. Dependencies required to run `libtock-rs`' tests (such as
  +`make`) are outside the scope of this document.
  +
  ## Unaudited Required Dependencies
  -`libtock-rs` has the following required dependencies, none of which are
  +`libtock-rs` has the following required build dependencies, none of which are
  currently audited:
  -* [`libcore`](https://github.com/rust-lang/rust/tree/master/src/libcore),
  - included as part of the Rust toolchain and implicitly added by the Rust
  - compiler.
  +* The Rust toolchain, including
  + [`cargo`](https://github.com/rust-lang/cargo),
  + [`rustc`](https://github.com/rust-lang/rust/tree/master/src/rustc), and
  + [`libcore`](https://github.com/rust-lang/rust/tree/master/src/libcore). The
  + specific toolchain version used is specified by the `rust-toolchain` file at
  + the root of the repository.
  * [`syn`](https://crates.io/crates/syn), pulled in by `libtock_codegen`.
  * [`quote`](https://crates.io/crates/quote), pulled in by `libtock_codegen`.
  * [`proc-macro2`](https://crates.io/crates/proc-macro2), pulled in by
  