Add codecov badge to README (#2049)

* Add codecov badge to README

* Update README.md

Co-authored-by: Disconnect3d <dominik.b.czarnota@gmail.com>

---------

Co-authored-by: Disconnect3d <dominik.b.czarnota@gmail.com>

README.md

@@ -3,6 +3,7 @@
 
 [![license](https://img.shields.io/github/license/mashape/apistatus.svg?maxAge=2592000)](https://choosealicense.com/licenses/mit/)
 [![Unit tests](https://github.com/pwndbg/pwndbg/actions/workflows/tests.yml/badge.svg?branch=dev&event=push)](https://github.com/pwndbg/pwndbg/actions/workflows/tests.yml)
+[![codecov.io](https://codecov.io/github/pwndbg/pwndbg/graph/badge.svg?token=i1cBPFVCav)](https://codecov.io/github/pwndbg/pwndbg?branch=dev)
 [![Discord](https://img.shields.io/discord/843809097920413717?label=Discord&style=plastic)](https://discord.gg/x47DssnGwm)
 
 `pwndbg` (/paʊnˈdiˌbʌɡ/) is a GDB plug-in that makes debugging with GDB suck less, with a focus on features needed by low-level software developers, hardware hackers, reverse-engineers and exploit developers.