Add link to Ubuntu notes about LTSEnablementStack

When running Ubuntu with a graphical environment, the kernel version MUST match the Xorg version. Only installing the suggested packages above breaks Unity for anything other than 2D mode (in VMware and probably native).
Signed-off-by: Ethan Spoelstra <ewspoelstra@computerphreaks.com>

docs/sources/installation/ubuntulinux.md

@@ -87,9 +87,18 @@ VirtualBox guest additions. If you didn't install the headers for your
 "precise" kernel, then you can skip these headers for the "raring"
 kernel. But it is safer to include them if you're not sure.
 
+Please read the installation instructions for backported kernels at
+Ubuntu.org to understand why you also need to install the Xorg packages
+if running Docker on a machine with a graphical environment like Unity.
+[LTS Enablement Stack](https://wiki.ubuntu.com/Kernel/LTSEnablementStack) refer to note 5 under
+each version.
+
     # install the backported kernel
     $ sudo apt-get update
     $ sudo apt-get install linux-image-generic-lts-raring linux-headers-generic-lts-raring
+    
+    # install the backported kernel and xorg if using Unity/Xorg
+    $ sudo apt-get install --install-recommends linux-generic-lts-raring xserver-xorg-lts-raring libgl1-mesa-glx-lts-raring
 
     # reboot
     $ sudo reboot