add new GPG key detailed in Docker installation
<p>completely new to 'Docker', wondered what this means in installation instructions.  <a href="https://docs.docker.com/engine/installation/linux/ubuntulinux/" rel="nofollow">https://docs.docker.com/engine/installation/linux/ubuntulinux/</a> </p>

<pre><code>4/ Add the new GPG key.
 sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --   recv-keys 58118E89F3A912897C070ADBF76221572C52609D
</code></pre>