docs: add build badge

Author: reliveyy

README.md

@@ -2,6 +2,7 @@
 
 [![Discord](https://img.shields.io/discord/547402601885466658.svg)](https://discord.gg/YgDhMSn)
 [![Go Report Card](https://goreportcard.com/badge/github.com/ExchangeUnion/xud-docker)](https://goreportcard.com/report/github.com/ExchangeUnion/xud-docker)
+[![Build](https://github.com/ExchangeUnion/xud-docker/workflows/Build/badge.svg)](https://github.com/ExchangeUnion/xud-docker/actions?query=workflow%3ABuild)
 [![Docker Pulls](https://img.shields.io/docker/pulls/exchangeunion/xud)](https://hub.docker.com/r/exchangeunion/xud)
 
 A complete [xud](https://github.com/ExchangeUnion/xud) environment targeted at market makers using [docker](https://www.docker.com/). Follow the 👉 [market maker guide](https://docs.exchangeunion.com/start-earning/market-maker-guide) 👈