Added NuGet badge to README

Author: GlebChili

README.md

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 # GmodSerilog
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+[![NuGet Link](https://img.shields.io/nuget/v/GmodNET.Serilog.Sink)](https://www.nuget.org/packages/GmodNET.Serilog.Sink/)
+
 A [Serilog](https://serilog.net/) Sink for Garry's Mod clients' and servers' consoles for [GmodDotNet](https://github.com/GmodNET/GmodDotNet) modules.
 
 ## Usage