docs: add codecov badge

Author: saitho

README.md

@@ -3,6 +3,7 @@
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/go-semantic-release/semantic-release)
 [![CI](https://github.com/saitho/git-file-webserver/workflows/CI/badge.svg?branch=master)](https://github.com/saitho/git-file-webserver/actions?query=workflow%3ACI+branch%3Amaster)
 [![Go Report Card](https://goreportcard.com/badge/github.com/saitho/git-file-webserver)](https://goreportcard.com/report/github.com/saitho/git-file-webserver)
+[![codecov](https://codecov.io/gh/saitho/git-file-webserver/branch/master/graph/badge.svg?token=BDDCKQG6DM)](https://codecov.io/gh/saitho/git-file-webserver)
 
 ## Features