Add installation procedure to README.md

Made some changes to the readme so it will make more sense if you've
downloaded the .zip from github.
Add SlugRock-vm.pk3

This .pk3 contains the compiled .qvm files from the SlugRock-code-test-4
branch.
Add SlugRock-media.pk3

This .pk3 contains the skill6.tga
Add SlugRock.bat

This .bat files contains the command line to start the SlugRock mod with
the ioquake3.exe from the SlugRock folder.