add: codecov badge

Author: mgxd

README.md

@@ -1,5 +1,6 @@
 # NiTransforms
 
 [![Build Status](https://travis-ci.org/poldracklab/nitransforms.svg?branch=master)](https://travis-ci.org/poldracklab/nitransforms)
+[![codecov](https://codecov.io/gh/poldracklab/nitransforms/branch/master/graph/badge.svg)](https://codecov.io/gh/poldracklab/nitransforms)
 
 A development repo for [nipy/nibabel#656](https://github.com/nipy/nibabel/pull/656)