Add Arch Linux example to the README.md
Add Arch Linux example to the README.md

There were examples of getting SDL2 development libraries on Ubuntu and Fedora. I've added an Arch Linux example.
Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,AngryLawyer/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2