Update README.md with cargo install instructions

Author: ifd3f

README.md

@@ -52,6 +52,7 @@ There are a couple of ways to install Caligula.
 - **Nix:**
   - [Nixpkgs](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/ca/caligula/package.nix): `nix-env -i caligula`
   - Repository flake: If your system is flake-enabled, you can use `github:ifd3f/caligula` for bleeding-edge changes.
+- **Cargo:** `cargo install caligula`
 - **Build from source:** This is a relatively standard cargo project so you should be able to just `git clone` and `cargo build --release` it.
 
 ### Platform support matrix