Add Rustup installation instructions to README

Now that cranelift is distributed with Rustup, I find myself constantly searching for the rust-lang/rust#81746 PR. I think that it would be useful to also describe the Rustup installation/usage instructions here in README.

Author: Kobzol

Readme.md

@@ -5,6 +5,34 @@ 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.
 
+## Download using Rustup
+The Cranelift codegen backend is now distributed in nightly builds on Linux and x86_64 macOS. If you want to
+install it using Rustup, you can do that by running:
+
+```bash
+$ rustup component add rustc-codegen-cranelift-preview --toolchain nightly
+```
+
+Once it is installed, you can enable it with one of the following approaches:
+- `CARGO_PROFILE_DEV_CODEGEN_BACKEND=cranelift cargo +nightly build -Zcodegen-backend`
+- `RUSTFLAGS="-Zcodegen-backend=cranelift" cargo +nightly build`
+- Add the following to `.cargo/config.toml`:
+    ```toml
+    [unstable]
+    codegen-backend = true
+
+    [profile.dev]
+    codegen-backend = "cranelift"
+    ```
+- Add the following to `Cargo.toml`:
+    ```toml
+    # This line needs to come before anything else in Cargo.toml
+    cargo-features = ["codegen-backend"]
+
+    [profile.dev]
+    codegen-backend = "cranelift"
+    ```
+
 ## Building and testing
 
 ```bash