README: add codecov badge

Author: blueyed

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/neovim/pynvim.svg?branch=master)](https://travis-ci.org/neovim/pynvim)
 [![Documentation Status](https://readthedocs.org/projects/pynvim/badge/?version=latest)](http://pynvim.readthedocs.io/en/latest/?badge=latest)
+[![Code coverage](https://codecov.io/gh/neovim/pynvim/branch/master/graph/badge.svg)](https://codecov.io/gh/neovim/pynvim)
 
 Pynvim implements support for python plugins in Nvim. It also works as a library for
 connecting to and scripting Nvim processes through its msgpack-rpc API.