Add simple OSX SDL2 mixer library install instructions.
Add simple OSX SDL2 mixer library install instructions.

Would have saved me some googling.
vadixidav/rust-sdl2,AngryLawyer/rust-sdl2,andelf/rust-sdl2_mixer,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,crumblingstatue/rust-sdl2_mixer,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2