Add alternative fud2 installation instructions

The existing instructions did not work to add `fud2` to my path (locally on macOS). `cargo install --path fud2` did.

docs/running-calyx/fud2/index.md

@@ -14,6 +14,8 @@ Until then, fud remains your first choice for all your build-related needs.
 fud2 is a Rust tool, so you can build it along with everything else in this monorepo with `cargo build`.
 You might then want to do something like ``ln -s `pwd`/target/debug/fud2 ~/.local/bin`` for easy access to the `fud2` binary.
 
+If you already have Calyx installed you can `cargo install --path fud2` from this repository's root to automatically add the binary to your path.
+
 fud2 depends on [Ninja][].
 Install it using your OS package manager or by downloading a binary.