chore: add codecov badge to README (#1204)

* chore: add codecov badge to README

* chore: move before callstack

Author: jaworek

README.md

@@ -12,6 +12,7 @@
 [![Version][version-badge]][package]
 [![PRs Welcome][prs-welcome-badge]][prs-welcome]
 [![Chat][chat-badge]][chat]
+![codecov](https://codecov.io/gh/callstack/react-native-testing-library/branch/main/graph/badge.svg?token=tYVSWro1IP)](https://codecov.io/gh/callstack/react-native-testing-library)
 [![Sponsored by Callstack][callstack-badge]][callstack]
 
 ## The problem