Must check "Create groups" when adding files to Xcode project

[gnestor]

When I added my sound files to my Xcode project the first time, I simply checked "Create folder references" vs. "Create groups" under the "Added folders" option. AudioPlayer.play wasn't playing any audio, so the sound files were not making it in the bundle. I tried adding the files again and checked "Create groups" and it worked. If this is not an isolated case, you should add this instruction to the README.