Add 21. Removed unneeded README.md file from 18 and 20.

This sample also demonstrates how to do mroe complicated cross-compile
for libraries that also have dependencies namely SDL_image and
SDL_mixer.

18_keyStates/README.md
20_forcefeedback/README.md
21_sfxMusic/.gitignore
21_sfxMusic/LButton.c
21_sfxMusic/LButton.h
21_sfxMusic/LTexture.c
21_sfxMusic/LTexture.h
21_sfxMusic/Makefile
21_sfxMusic/README.md
21_sfxMusic/beat.wav
21_sfxMusic/bensound-ukulele.mp3
21_sfxMusic/bensound-ukulele.ogg
21_sfxMusic/common.c
21_sfxMusic/common.h
21_sfxMusic/high.wav
21_sfxMusic/libmpg123.dylib
21_sfxMusic/libvorbisfile.dylib
21_sfxMusic/low.wav
21_sfxMusic/medium.wav
21_sfxMusic/prompt.png
21_sfxMusic/scratch.wav
21_sfxMusic/sfxmusic.c
21_sfxMusic/sfxmusic.exe

--

Successfully tested cross-compile result on Windows 32 bit on top of VirtualBox, and on dev machine macOS. Works fine. But hardware acceleration support on virtualbox makes it slower to run.