docs: add codecov

Author: KyLeoHC

README.md

@@ -1,5 +1,6 @@
 <a href="https://www.npmjs.com/package/inline-source-webpack-plugin"><img src="https://img.shields.io/npm/v/inline-source-webpack-plugin.svg" alt="npm Version"></a>
 [![Build Status](https://travis-ci.org/KyLeoHC/inline-source-webpack-plugin.svg?branch=master)](https://travis-ci.org/KyLeoHC/inline-source-webpack-plugin)
+![Codecov](https://img.shields.io/codecov/c/github/KyLeoHC/inline-source-webpack-plugin)
 
 # inline-source-webpack-plugin