I want to make a game using Armory using WebAssembly Rust as shown in the link.
I believe I have to first compile Rust to WebAssembly then I can execute the code.
Not finding a very good source on how to compile from Rust to WebAssembly. I have already installed wasm by typing on cmd cargo install wasm-pack on Windows.
Now what do I do from here? How do I compile Rust to WASM?
The Rust & WebAssembly working group has extensive material to get you started with it. Check out the book on Rust 
and WebAssembly 
Whats the one command to compile Rust to WASM?
cargo build --release --target wasm32-unknown-unknown after you've installed WASM target with rustup target add wasm32-unknown-unknown. But you will have to manually mark functions with pub extern and #[no_mangle], and write a certain amount of JS boilerplate. See for example this small demo: Demo: Update a canvas from wasm - Hello, Rust!
But probably you don't want such low-level approach, if so follow the link posted by @jer.