Add aqua badge (#257)

Author: aurorarossi

README.md

@@ -4,6 +4,7 @@
 [![Build status](https://github.com/JuliaGraphs/Graphs.jl/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/JuliaGraphs/Graphs.jl/actions/workflows/ci.yml?query=branch%3Amaster)
 [![Code coverage](http://codecov.io/github/JuliaGraphs/Graphs.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaGraphs/Graphs.jl?branch=master)
 [![Code style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)
+[![Aqua QA](https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg)](https://github.com/JuliaTesting/Aqua.jl)
 
 ## Overview