Documentation to compile Retro

[roncli]

Let's get some instructions on how to compile Retro for all major environments.

See #82 for Mac. Need Windows and Linux.

[sdfgeoff]

On Archlinux,

# Get dependencies:
sudo pacman -S sdl sdl_mixer mesa physfs scons unzip
# Get sources:
git clone https://github.com/CDarrow/DXX-Retro.git
cd DXX-Retro

# build:
cd d1; scons
cd ../d2; scons

Process on Ubuntu/debian derivatives will be similar, but the dependencies will have different names. I don't currently have a ubuntu partition to play with to figure out what they are.

[roncli]

@sdfgeoff Why mesa and unzip? I don't use those on the Mac build.

[sdfgeoff]

I just grabbed the deps from the rebirth AUR:
https://aur.archlinux.org/packages/d1x-rebirth/
and:
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=d1x-rebirth

I assume that the maintainer had some reason for picking them. Actually, it looks like unzip may not be needed. I guess I should actually test without them (trying to remove unzip without uninstalling other packages will be fun...)