Add Codecov badge

Author: Geod24

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.com/geod24/bitblob.svg?branch=v1.x.x)](https://travis-ci.com/geod24/bitblob)
 [![DUB Package](https://img.shields.io/dub/v/bitblob.svg)](https://code.dlang.org/packages/bitblob)
+[![codecov](https://codecov.io/gh/Geod24/bitblob/branch/v1.x.x/graph/badge.svg)](https://codecov.io/gh/Geod24/bitblob)
 
 A lightweight wrapper to represent hashes as value types.