Add stable docs badge (#6)

Author: gdalle

README.md

@@ -1,6 +1,7 @@
 # SparseMatrixColorings.jl
 
 [![Build Status](https://github.com/gdalle/SparseMatrixColorings.jl/actions/workflows/Test.yml/badge.svg?branch=main)](https://github.com/gdalle/SparseMatrixColorings.jl/actions/workflows/Test.yml?query=branch%3Amain)
+[![Stable Documentation](https://img.shields.io/badge/docs-stable-blue.svg)](https://gdalle.github.io/SparseMatrixColorings.jl/stable/)
 [![Dev Documentation](https://img.shields.io/badge/docs-dev-blue.svg)](https://gdalle.github.io/SparseMatrixColorings.jl/dev/)
 [![Coverage](https://codecov.io/gh/gdalle/SparseMatrixColorings.jl/branch/main/graph/badge.svg)](https://app.codecov.io/gh/gdalle/SparseMatrixColorings.jl)
 [![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/JuliaDiff/BlueStyle)