README / update OSX docker client install

If you want to use the brand new Docker OSX client:

```
$ curl http://get.docker.io/builds/Darwin/x86_64/docker-latest.tgz | tar xvz
$ chmod +x ./usr/local/bin/docker
$ export DOCKER_HOST=localhost
$ ./usr/local/bin/docker version
$ sudo cp ./usr/local/bin/docker /usr/local/bin/
```