Add build instructions to README

README.md

@@ -8,6 +8,16 @@ This repository contains the source code for the [Minimal Kernel][post] post of
 
 **Check out the [master branch](https://github.com/phil-opp/blog_os) for more information.**
 
+## Building
+
+- Install the `x86_64-unknown-none` target using rustup:
+  ```
+  rustup target add x86_64-unknown-none
+  ```
+- Build by running:
+  ```
+  cargo build --target x86_64-unknown-none
+  ```
 
 ## License