docs: Add virt-lightning app

There was this introduction made on the users list:

https://www.redhat.com/archives/libvirt-users/2019-March/msg00046.html

Add the application onto the list of apps known to use libvirt.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>

docs/apps.html.in

@@ -99,6 +99,12 @@
         machines. It is a command line tool for developers that makes it very
         fast and easy to deploy and re-deploy an environment of vm's.
       </dd>
+      <dt><a href="https://github.com/virt-lightning/virt-lightning">virt-lightning</a></dt>
+      <dd>
+        Virt-Lightning uses libvirt, cloud-init and libguestfs to allow anyone
+        to quickly start a new VM. Very much like a container CLI, but with a
+        virtual machine.
+      </dd>
     </dl>
 
     <h2><a id="configmgmt">Configuration Management</a></h2>