INSTALLATION.md: Directly link to install instructions, not homepage
After the 2018 Edition the Rust site has been redesigned with the
"Install Rust" link removed.
To avoid confusion this PR includes a direct link to the installation
instructions.
-Methods for installing Rust change as the language evolves. To get up-to-date installation instructions head to the [Rust homepage](https://www.rust-lang.org/) and click the "Install Rust" link.
+Methods for installing Rust change as the language evolves. To get up-to-date installation instructions head to the [Rust install page](https://www.rust-lang.org/tools/install).
