Add source graph used by badge
We are used by 10+ other projects now. Think it makes
sense to add this badge.
[![Go Report Card](https://goreportcard.com/badge/github.com/coredns/coredns)](https://goreportcard.com/report/coredns/coredns)
[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcoredns%2Fcoredns.svg?type=shield)](https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcoredns%2Fcoredns?ref=badge_shield)
[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1250/badge)](https://bestpractices.coreinfrastructure.org/projects/1250)
+[![Sourcegraph](https://sourcegraph.com/github.com/coredns/coredns/-/badge.svg)](https://sourcegraph.com/github.com/coredns/coredns?badge)
CoreDNS (written in Go) chains [plugins](https://coredns.io/plugins). Each plugin performs a DNS
function.
