Re-Add Rustup to the README (#648)

In the beginning, there were clear instructions for users that had the problems addressed by NativeLink but were not necessarily Rust users. Rust is an implementation detail and Rust knowledge cannot be a pre-requisite.

README.md

@@ -19,6 +19,11 @@ Below, you will find a few different options for getting started with Native Lin
 
 ### 📦 Installing with Cargo
 
+1. First install Rust, but skip to step 2 if you have it already.
+```bash
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+```
+2. Install NativeLink with Cargo.
 ```bash
 cargo install --git https://github.com/TraceMachina/nativelink --tag v0.2.0
 ```