add stable docs badge

Author: tjjarvinen

README.md

@@ -1,5 +1,6 @@
 # AtomsSystems.jl
 
+[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://JuliaMolSim.github.io/AtomsSystems.jl/stable)
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://JuliaMolSim.github.io/AtomsSystems.jl/dev)
 [![Build Status](https://github.com/JuliaMolSim/AtomsSystems.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/JuliaMolSim/AtomsSystems.jl/actions/workflows/CI.yml?query=branch%3Amain)
 [![codecov](https://codecov.io/gh/JuliaMolSim/AtomsSystems.jl/graph/badge.svg?token=QPK831PYGJ)](https://codecov.io/gh/JuliaMolSim/AtomsSystems.jl)