Rough documentation for the update process of 3p Rust crates.

TO-174 #comment process docs

Change-Id: I8873cd0a7e1a5b7021afc5938fc8fd5d137a669e

rust.md

@@ -15,5 +15,29 @@ Note that both targets can be used with the Fuchsia toolchain and with the host
 toolchain.
 
 
+# Managing third-party dependencies
+
+Third-party crates are stored in [`//third-party/rust-crates/vendor`][3p-crates]
+which we use as a [directory source][source-replacement].
+
+To be able to run the update script, you first need to install the
+`cargo-vendor` utility:
+its dependencies lack the proper licenses.
+```
+scripts/install_cargo_vendor.sh
+```
+
+To update these crates, run the following commands:
+```
+scripts/update_rust_packages.py
+build/rust/fetch-vendor-license.py --directory third_party/rust-crates/vendor
+```
+The configurations used as a reference to generate the set of required crates
+are listed in the [`update_rust_packages.py`][update-script] script.
+
+
 [target-library]: https://fuchsia.googlesource.com/build/+/master/rust/rust_library.gni "Rust library"
 [target-binary]: https://fuchsia.googlesource.com/build/+/master/rust/rust_binary.gni "Rust binary"
+[3p-crates]: https://fuchsia.googlesource.com/third_party/rust-crates/+/master/vendor "Third-party crates"
+[source-replacement]: http://doc.crates.io/source-replacement.html "Source replacement"
+[update-script]: https://fuchsia.googlesource.com/scripts/+/master/update_rust_packages.py "Update script"