Add performance notes to the README [Skip CI]

Author: nblumhardt

README.md

@@ -146,6 +146,12 @@ To write events to the file in an alternative format such as JSON, pass an `ITex
     .WriteTo.RollingFile(new JsonFormatter(), "log-{Date}.txt")
 ```
 
+### Performance
+
+By default, the rolling file sink will flush each event written through it to disk. To improve write performance, specifying `buffered: true` will permit the underlying stream to buffer writes.
+
+The [Serilog.Sinks.Async](https://github.com/serilog/serilog-sinks-async) package can be used to wrap the rolling file sink and perform all disk accss on a background worker thread.
+
 ### Alternatives
 
 The default rolling file sink is designed to suit most applications. So that we can keep it maintainable and reliable, it does not provide a large range of optional behavior. Check out alternative implemementations like [this one](https://github.com/BedeGaming/sinks-rollingfile) if your needs aren't met by the default version.