Add discord community (#181)

* Update README.md

* Update README.md

Author: lukas-h

README.md

@@ -12,6 +12,8 @@
 ![dart publish](https://github.com/dartclub/turf_dart/actions/workflows/dart-pub-publish.yml/badge.svg)
 ![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)
 
+<h3>–> <a href="https://discord.gg/cTJk98cU">Join our Dart / Flutter GIS Community on Discord</a> <–</h3>
+
 TurfDart is a Dart library for [spatial analysis](https://en.wikipedia.org/wiki/Spatial_analysis). It includes traditional spatial operations, helper functions for creating GeoJSON data, and data classification and statistics tools. You can use TurfDart in your Flutter applications on the web, mobile and desktop or in pure Dart applications running on the server.
 
 As the foundation, we are using [Geotypes](https://github.com/dartclub/geotypes), a lightweight dart library that provides a strong GeoJSON object model and fully [RFC 7946](https://tools.ietf.org/html/rfc7946) compliant serializers.