Update README.md

Author: i-make-robots

README.md

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 [![Release](https://jitpack.io/v/MarginallyClever/NodeGraphCore.svg)](https://jitpack.io/#MarginallyClever/NodeGraphCore)
 ![workflow](https://github.com/MarginallyClever/NodeGraphCore/actions/workflows/main.yml/badge.svg)
+[![Discord](https://img.shields.io/discord/521753686238560256?label=Discord)](https://discord.gg/QtvHqAv8yp)
 
 A pure Java implementation of [Flow-based Programming](https://en.wikipedia.org/wiki/Dataflow_programming) (FBP).