Update build instructions

Author: bjorn3

Readme.md

@@ -4,7 +4,7 @@
 
 The goal of this project is to create an alternative codegen backend for the rust compiler based on [Cranelift](https://github.com/bytecodealliance/wasmtime/blob/master/cranelift). This has the potential to improve compilation times in debug mode. If your project doesn't use any of the things listed under "Not yet supported", it should work fine. If not please open an issue.
 
-## Building
+## Building and testing
 
 ```bash
 $ git clone https://github.com/bjorn3/rustc_codegen_cranelift.git
@@ -13,6 +13,13 @@ $ ./prepare.sh # download and patch sysroot src and install hyperfine for benchm
 $ ./test.sh --release
 ```
 
+If you want to only build but not test you should replace the last command with:
+
+```bash
+$ cargo build --release
+$ ./build_sysroot/build_sysroot.sh
+```
+
 ## Usage
 
 rustc_codegen_cranelift can be used as a near-drop-in replacement for `cargo build` or `cargo run` for existing projects.