Ditch appveyor due to proper .NET Core 3 support

README.md

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-# Wake-On-LAN [![Travis Build Status](https://travis-ci.org/nikeee/wake-on-lan.svg?branch=master)](https://travis-ci.org/nikeee/wake-on-lan) [![Windows Build Status](https://ci.appveyor.com/api/projects/status/2t6mkn484amt0j4o?svg=true)](https://ci.appveyor.com/project/nikeee/wake-on-lan) [![NuGet version](https://img.shields.io/nuget/v/WakeOnLan.svg)](https://nuget.org/packages/WakeOnLan)
+# Wake-On-LAN [![Travis Build Status](https://travis-ci.org/nikeee/wake-on-lan.svg?branch=master)](https://travis-ci.org/nikeee/wake-on-lan) [![NuGet version](https://img.shields.io/nuget/v/WakeOnLan.svg)](https://nuget.org/packages/WakeOnLan)
 
 A simple library for sending magic packets and performing IP address operations. The default namespace is `System.Net`. There is an online documentation available [here](https://nikeee.github.io/wake-on-lan/README.html).