Update build.md
Update build.md

Add information about downloading the Skia native libraries via the supplied script.

wieslawsoltes/Perspex,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,jazzay/Perspex,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,MrDaedra/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,Perspex/Perspex,Perspex/Perspex,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,OronDF343/Avalonia,susloparovdenis/Avalonia,punker76/Perspex,susloparovdenis/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,MrDaedra/Avalonia,OronDF343/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,susloparovdenis/Perspex,jkoritzinsky/Avalonia,susloparovdenis/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,akrisiun/Perspex,SuperJMN/Avalonia,grokys/Perspex,grokys/Perspex