Add Docker Hub link to automated builds

Author: tgerring

README.md

@@ -29,6 +29,7 @@ For further, detailed, build instruction please see the [Wiki](https://github.co
 Automated (dev) builds
 ======================
 
+* [[Docker](https://registry.hub.docker.com/u/ethereum/client-go/)]
 * [[OS X](http://build.ethdev.com/builds/OSX%20Go%20develop%20branch/Mist-OSX-latest.dmg)]
 * [Windows] Coming soon™
 * [Linux] Coming soon™