Add code style badge

Author: nickrobinson251

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl.svg?branch=master)](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl)
 [![codecov.io](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl/coverage.svg?branch=master)](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl?branch=master)
 [![Latest Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliadiff.github.io/FiniteDifferences.jl/latest/)
+[![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)
 
 FiniteDifferences.jl estimates derivatives with [finite differences](https://en.wikipedia.org/wiki/Finite_difference).