Add a way to execute async

[javis86]

I suggest include in the documentation access to an async way to publish messages to Discord. The way i found is using Async Sink from serilog-sinks-async:

 Log.Logger = new LoggerConfiguration()
                                .WriteTo.Async( a => 
                                    a.Discord(11223344556677, "xxxxxxxxxxxxx"))
                                .Enrich.FromLogContext()
                                .CreateLogger();
`

[AblEdge]

thank you @javis86 Ill take a look