Add source graph used by badge (#1209)
Add source graph used by badge (#1209)

We are used by 10+ other projects now. Think it makes
sense to add this badge.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
jremond/coredns,coredns/coredns,yongtang/coredns,miekg/coredns,chrisohaver/coredns,johnbelamaric/coredns,ii/coredns,jremond/coredns,ii/coredns,johnbelamaric/coredns,miekg/coredns