Add coverage badge. (#113123)

We are finally generating coverage reports on post submit for the flutter repository.
This is adding the badge to the README file of the repository.

README.md

@@ -10,6 +10,7 @@
 [![Build Status - Cirrus][]][Build status]
 [![Discord badge][]][Discord instructions]
 [![Twitter handle][]][Twitter badge]
+[![codecov](https://codecov.io/gh/flutter/flutter/branch/master/graph/badge.svg?token=11yDrJU2M2)](https://codecov.io/gh/flutter/flutter)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5631/badge)](https://bestpractices.coreinfrastructure.org/projects/5631)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/flutter/flutter/badge)](https://api.securityscorecards.dev/projects/github.com/flutter/flutter)