Added codecov badge

Author: cmaglie

README.md

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-# go.bug.st/relaxed-semver [![build status](https://api.travis-ci.org/bugst/relaxed-semver.svg?branch=master)](https://travis-ci.org/bugst/relaxed-semver)
+# go.bug.st/relaxed-semver [![build status](https://api.travis-ci.org/bugst/relaxed-semver.svg?branch=master)](https://travis-ci.org/bugst/relaxed-semver) [![codecov](https://codecov.io/gh/bugst/relaxed-semver/branch/master/graph/badge.svg)](https://codecov.io/gh/bugst/relaxed-semver)
+
 
 
 A library for handling a superset of semantic versioning in golang.