Use logo as the title of the README.md (#911)

The logo is pointing to https://coredns.io/images/CoreDNS_Colour_Horizontal.png
and provides the link to https://coredns.ioSigned-off-by: Yong Tang <yong.tang.github@outlook.com>

README.md

-# CoreDNS
+[![CoreDNS](https://coredns.io/images/CoreDNS_Colour_Horizontal.png)](https://coredns.io)
 
 [![Documentation](https://img.shields.io/badge/godoc-reference-blue.svg)](https://godoc.org/github.com/coredns/coredns)
 [![Build Status](https://img.shields.io/travis/coredns/coredns/master.svg?label=build)](https://travis-ci.org/coredns/coredns)