Add instructions of how to install SDL2 through package manager to README

[bvssvni]

I have seen this question come up a couple of times.

If you try to install SDL2 by downloading the developer libraries and copy to /Library/Frameworks, then it won't work when building with Cargo because it does not detect this version.

As most new users will try to use Cargo, it would be nice to add instructions of how to install SDL2 through brew and where to get brew if they don't already have it.

[AngryLawyer]

Sure, if anyone wants to write instructions (I've always built from source) I'll be happy to merge it in

[bvssvni]

I think it is:

brew install sdl2

[bvssvni]

For rust-sdl2_mixer it is:

brew install sdl2_mixer

[bvssvni]

I've written a post on Rust-Empty PistonDevelopers/rust-empty#175

I think it would suffice to link to the issue from README if people experience problems with using brew to install SDL2.

[AngryLawyer]

There we go

[bvssvni]

Nice!

[AngryLawyer]

:)