add codecov badge (apache-superset#39)

Since it's 100% it seems worthwhile to add ;)

Author: williaster

README.md

@@ -1,5 +1,6 @@
 # @superset-ui
 
+[![Codecov](http://codecov.io/github/apache-superset/superset-ui/coverage.svg?branch=master)](http://codecov.io/github/apache-superset/superset-ui/coverage.svg?branch=master)
 [![Build Status](https://img.shields.io/travis/com/apache-superset/superset-ui/master.svg?style=flat-square
 )](https://travis-ci.com/apache-superset/superset-ui)
 [![David](https://img.shields.io/david/dev/apache-superset/superset-ui.svg?style=flat-square)](https://david-dm.org/apache-superset/superset-ui?type=dev)