Update README.md

Author: garyb

README.md

@@ -2,6 +2,7 @@
 
 [![Latest release](http://img.shields.io/github/release/purescript/purescript-sets.svg)](https://github.com/purescript/purescript-sets/releases)
 [![Build status](https://travis-ci.org/purescript/purescript-sets.svg?branch=master)](https://travis-ci.org/purescript/purescript-sets)
+[![Dependency status](https://img.shields.io/librariesio/github/purescript/purescript-sets.svg)](https://libraries.io/github/purescript/purescript-sets)
 
 Purely-functional set data structure