README.md: added additional information about supporting os
README.md: added additional information about supporting os

changed the statement "applies wallpaper in a gnome desktop" to "applies wallpaper on your desktop. Works with windows, mac and gtk based desktops on Linux".

Fixes #78

mohitshaw/UIP,NIT-dgp/UIP,VK10/UIP,hackrush01/UIP,DarkSouL11/UIP,nemaniarjun/UIP,Aniq55/UIP,hassi2016/UIP,Aniq55/UIP,akshatnitd/UIP,NIT-dgp/UIP,VK10/UIP,nemaniarjun/UIP