Update README.md

PyPi statistics badge. Cool!

Author: ianamason

README.md

@@ -2,6 +2,7 @@
 ==================
 [![PyPI version](https://badge.fury.io/py/wllvm.svg)](https://badge.fury.io/py/wllvm)
 [![Build Status](https://travis-ci.org/SRI-CSL/whole-program-llvm.svg?branch=master)](https://travis-ci.org/SRI-CSL/whole-program-llvm)
+[![PyPI Statistics](https://img.shields.io/pypi/dm/wllvm.svg)](https://pypistats.org/packages/wllvm)
 
 
 Introduction