Add codecov badge

Signed-off-by: Matt Lord <[email protected]>

README.md

@@ -1,5 +1,6 @@
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.vitess/vitess-jdbc/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.vitess/vitess-jdbc)
 [![codebeat badge](https://codebeat.co/badges/51c9a056-1103-4522-9a9c-dc623821ea87)](https://codebeat.co/projects/github-com-youtube-vitess)
+[![Coverage Status](https://codecov.io/gh/vitessio/vitess/branch/main/graph/badge.svg)](https://codecov.io/gh/vitessio/vitess)
 [![Go Report Card](https://goreportcard.com/badge/vitess.io/vitess)](https://goreportcard.com/report/vitess.io/vitess)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fvitessio%2Fvitess.svg?type=shield)](https://app.fossa.io/projects/git%2Bgithub.com%2Fvitessio%2Fvitess?ref=badge_shield)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1724/badge)](https://bestpractices.coreinfrastructure.org/projects/1724)