message: Update the rustc usage section of the readme
diff: @@ -25,8 +25,10 @@ If you compiled cg_clif in debug mode you should use `CHANNEL="debug"` instead o
  ### Rustc
  +> You should prefer using the Cargo method.
  +
  ```bash
  -$ rustc -Cpanic=abort -Zcodegen-backend=$cg_clif_dir/target/release/librustc_codegen_cranelift.so --sysroot $cg_clif_dir/build_sysroot/sysroot my_crate.rs
  +$ rustc +$(cat $cg_clif_dir/rust-toolchain) -Cpanic=abort -Zcodegen-backend=$cg_clif_dir/target/release/librustc_codegen_cranelift.so --sysroot $cg_clif_dir/build_sysroot/sysroot my_crate.rs
  ```
  ## Env vars
  