Update README.md

Author: mivano

README.md

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 This repository contains two nuget packages: `Serilog.Sinks.Elasticsearch` and `Serilog.Formatting.Elasticsearch`.
 
+> __Just a heads up that the .NET team @elastic have created their own new Serilog Sink called Elastic.Serilog.Sinks (Package: https://www.nuget.org/packages/Elastic.Serilog.Sinks#readme-body-tab and documentation: https://www.elastic.co/guide/en/ecs-logging/dotnet/current/serilog-data-shipper.html). Althought this current sink will still work, I advice you to have a look first at the official Elastic implementation as it is better supported and more up to date.__
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 ## Table of contents
 
 * [What is this sink](#what-is-this-sink)