Add gitter badge to README.md

Author: mellinoe

README.md

@@ -8,6 +8,8 @@ This wrapper is built on top of [cimgui](https://github.com/Extrawurst/cimgui),
 
 [![NuGet](https://img.shields.io/nuget/v/ImGui.NET.svg)](https://www.nuget.org/packages/ImGui.NET)
 
+[![Join the chat at https://gitter.im/ImGui-NET/Lobby](https://badges.gitter.im/ImGui-NET/Lobby.svg)](https://gitter.im/ImGui-NET/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 # Building
 
 ImGui.NET can be built in Visual Studio or on the command line. The .NET Core SDK is needed to build on the command line, and it can be downloaded [here](https://www.microsoft.com/net/core). Visual Studio 2017 is the minimum VS version supported for building.