Add documentation describing `libtock-rs`'s unaudited third-party dependencies, as required by Tock's threat model.

doc/Dependencies.md

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+Third Party Dependencies
+========================
+
+## Unaudited Required Dependencies
+
+`libtock-rs` has the following required 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.
+* [`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
+  `libtock_codegen`.
+
+## Avoiding Optional Dependencies
+
+To avoid pulling in optional dependencies, users should use `libtock-core`
+instead of `libtock`. `libtock-core` is in the `core/` directory.