docs: restore Codecov badge in README

The badge was accidentally removed from README.md in commit
3edaeaf (which was only supposed to remove it from the GitHub
Pages landing page). The user wants it present in the README
as a project-health signal.

Author: gilbertwong96

README.md

@@ -14,6 +14,7 @@ Built with [Finch](https://hex.pm/packages/finch),
 [Burrito](https://hex.pm/packages/burrito).
 
 [![CI](https://github.com/gilbertwong96/ado_cli/actions/workflows/ci.yml/badge.svg)](https://github.com/gilbertwong96/ado_cli/actions/workflows/ci.yml)
+[![codecov](https://codecov.io/gh/gilbertwong96/ado_cli/graph/badge.svg)](https://codecov.io/gh/gilbertwong96/ado_cli)
 [![Elixir](https://img.shields.io/badge/elixir-1.20+-purple.svg)](https://elixir-lang.org)
 [![License: Apache-2.0](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![Website](https://img.shields.io/badge/website-gilbertwong96.github.io%2Fado_cli-blue)](https://gilbertwong96.github.io/ado_cli/)