docs(README): add Code Coverage badge

Author: hutson

README.md

@@ -1,6 +1,7 @@
 # conventional-github-releaser
 
 [![Build Status](https://travis-ci.org/conventional-changelog/conventional-github-releaser.svg?branch=master)](https://travis-ci.org/conventional-changelog/conventional-github-releaser)
+[![codecov.io](https://codecov.io/github/conventional-changelog/conventional-github-releaser/coverage.svg?branch=master)](https://codecov.io/github/conventional-changelog/conventional-github-releaser?branch=master)
 [![Join the chat at https://gitter.im/conventional-changelog/conventional-github-releaser](https://badges.gitter.im/conventional-changelog/conventional-github-releaser.svg)](https://gitter.im/conventional-changelog/conventional-github-releaser?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 > Make a new GitHub release from git metadata.