Add License Scanning badge (#663)

In CNCF, we are experimenting with FOSSA for license scanning on all of our projects.

https://app.fossa.io/reports/7d989803-8931-4221-a11f-330b7f333cdd

README.md

@@ -4,6 +4,7 @@
 [![Build Status](https://img.shields.io/travis/coredns/coredns.svg?style=flat-square&label=build)](https://travis-ci.org/coredns/coredns)
 [![Code Coverage](https://img.shields.io/codecov/c/github/coredns/coredns/master.svg?style=flat-square)](https://codecov.io/github/coredns/coredns?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/coredns/coredns?style=flat-square)](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)
 
 CoreDNS is a DNS server that started as a fork of [Caddy](https://github.com/mholt/caddy/). It has
 the same model: it chains middleware. In fact it's so similar that CoreDNS is now a server type