geckodriver: add link to rustup toolchain website

Source-Repo: https://github.com/mozilla/geckodriver
Source-Revision: fdf9fbebff651bac93e6beba3d3941c3447d64db

--HG--
extra : rebase_source : 56de8d6c103c4526a23dbb7a4fb6aa7927334c95

testing/geckodriver/README.md

@@ -81,7 +81,7 @@ contain any of the following fields:
 ## Building
 
 geckodriver is written in [Rust](https://www.rust-lang.org/)
-and you need the Rust toolchain to compile it.
+and you need the [Rust toolchain](https://rustup.rs/) to compile it.
 
 To build the project for release,
 ensure you do a compilation with optimisations: