Add "Code Style: Blue" badge to README

Author: goerz

README.md

@@ -4,6 +4,7 @@
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://JuliaCollections.github.io/Bijections.jl/dev/)
 [![CI](https://github.com/JuliaCollections/Bijections.jl/actions/workflows/CI.yml/badge.svg?branch=master)](https://github.com/JuliaCollections/Bijections.jl/actions/workflows/CI.yml)
 [![Coverage](https://codecov.io/gh/JuliaCollections/Bijections.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaCollections/Bijections.jl)
+[![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/JuliaDiff/BlueStyle)
 
 Julia dictionaries are not one-to-one. That is, two different keys might have the same value.
 A `Bijection` data structure behaves just like a dictionary, except it prevents the assignment of the same value to two different keys.