Update readme.md
Update readme.md

We don't maintain this build feed anymore so the badge is redundant.
AvaloniaUI/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,grokys/Perspex,AvaloniaUI/Avalonia,grokys/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia