[README] Add critical instructions to README

.. and point to the canonical docs for more information.

Change-Id: I94754b679f37644bc771fb46bb4130664237d34d
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-Fuchsia Rust package (crates.io) mirror Repository
-==================================================
+# Fuchsia Rust package (crates.io) mirror Repository
 
-This repository contains a copy of Rust packages used in Fuchsia, fetched from
-[crates.io](https://crates.io/). It was generated by running the
-[cargo-vendor](https://github.com/alexcrichton/cargo-vendor) tool over all
-targets in the Fuchsia tree, with the `--explicit-version` flag specified so
-that subdirectory names contain the version.
+## Quickstart
 
-In cases where the crate hosted by crates.io does not contain a LICENSE file
-(see [cargo issue #3537](https://github.com/rust-lang/cargo/issues/3537) for
-why this happens), a script fetched the license(s) from the upstream
-repository and wrote them to LICENSE at the root of each crate.
+After modifying `Cargo.toml`,
+Run `fx update-rustc-third-party`
+
+## macOS
+
+You will need a modern OpenSSL and to export either `PKG_CONFIG_PATH` or `CFLAGS` and `LDFLAGS` in order for cargo to find it, before running `fx update-rustc-third-party`.
+
+You can install using `brew install openssl` and observe `brew info openssl` for the above exports. It is recommended you only export these as-needed, not permanently, to avoid causing incompatibilities with other software.
+
+## Description
+
+This repository contains vendored copies of third party code used in Fuchsia.
+
+Further documentation is available here:
+
+https://fuchsia.googlesource.com/docs/+/master/development/languages/rust/third_party.md
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