Add docs stable badge (#348)

Author: gdalle

README.md

@@ -1,6 +1,7 @@
 # Graphs.jl
 
-[![Documentation](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliagraphs.org/Graphs.jl/dev/)
+[![Documentation stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliagraphs.org/Graphs.jl/stable/)
+[![Documentation dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliagraphs.org/Graphs.jl/dev/)
 [![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)