feat: add coverage badge

Author: jtr109

README.md

@@ -4,6 +4,7 @@
 [![Release](https://github.com/jtr109/percent-encoding-command/workflows/Release/badge.svg)](https://github.com/jtr109/percent-encoding-command/actions?query=workflow%3ARelease)
 [![MIT License](https://img.shields.io/crates/l/percent-encoding-command)](https://github.com/jtr109/percent-encoding-command/blob/master/LICENSE-MIT)
 [![Apache 2.0 License](https://img.shields.io/crates/l/percent-encoding-command)](https://github.com/jtr109/percent-encoding-command/blob/master/LICENSE-APACHE)
+[![codecov](https://codecov.io/gh/jtr109/percent-encoding-command/branch/master/graph/badge.svg?token=TWDD8V3LJG)](https://codecov.io/gh/jtr109/percent-encoding-command)
 
 A command for percent-encoding conversion.