Longer README about how to use this instead of cargo run (rust-lang#1054)

* Longer README about how to use this instead of `cargo run`

Resolves rust-lang#1052.

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

Author: vi

Readme.md

@@ -13,14 +13,20 @@ $ ./test.sh --release
 
 ## Usage
 
-`$cg_clif_dir` is the directory you cloned this repo into in the following instructions.
+rustc_codegen_cranelift can be used as a near-drop-in replacement for `cargo build` or `cargo run` for existing projects.
+
+Assuming `$cg_clif_dir` is the directory you cloned this repo into and you followed the instructions (`prepare.sh` and `test.sh`).
 
 ### Cargo
 
+In the directory with your project (where you can do the usual `cargo build`), run:
+
 ```bash
 $ CHANNEL="release" $cg_clif_dir/cargo.sh run
 ```
 
+This should build and run your project with rustc_codegen_cranelift instead of the usual LLVM backend.
+
 If you compiled cg_clif in debug mode (aka you didn't pass `--release` to `./test.sh`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely.
 
 ### Rustc