Added link to CONTRIBUTING.md in README.md
Added link to CONTRIBUTING.md in README.md

bright-sparks/android-UniversalMusicPlayer,eneim/android-UniversalMusicPlayer,xCatG/android-UniversalMusicPlayer,cartland/android-UniversalMusicPlayer,android/uamp,MaTriXy/android-UniversalMusicPlayer,android/uamp