Update README [Skip CI]

Author: nblumhardt

README.md

@@ -22,10 +22,19 @@ The formatter is used in conjunction with sinks that accept `ITextFormatter`. Fo
 
 ```csharp
 Log.Logger = new LoggerConfiguration()
-  .WriteTo.Sink(new RollingFileSink("./logs/myapp.txt", new CompactJsonFormatter(), null, null))
+  .WriteTo.RollingFile(new CompactJsonFormatter(), "./logs/myapp.json")
   .CreateLogger();
 ```
 
+To specify the formatter in XML `<appSettings>` provide its assembly-qualified type name:
+
+```xml
+<appSettings>
+  <add key="serilog:write-to:RollingFile.pathFormat" value="./logs/myapp.json" />
+  <add key="serilog:write-to:RollingFile.formatter"
+       value="Serilog.Formattting.Compact.CompactJsonFormatter, Serilog.Formatting.Compact" />
+```
+
 ### Rendered events
 
 `CompactJsonFormatter` will preserve the message template, properties, and formatting information so that the rendered message can be created at a later point. When the