Update user docs to cover custom distro

The custom distro feature was added to beaker,
https://github.com/beaker-project/beaker-dev-docs/blob/master/proposals/allow-installation-user-defined-distro.rst
But it was never documented.  This PR documents that and adds
documentation for the image node which allows further customization by
allowing you to specify the boot loader used for network installs.

documentation/user-guide/job-xml.rst

@@ -163,8 +163,12 @@ using the ``<watchdog/>`` element. If you want to disable panic detection, for
 example because your tests are expecting to trigger a kernel panic, add an 
 attribute ``panic="ignore"`` to the ``<watchdog/>`` element.
 
-To actually determine what distro will be installed, the
-``<distroRequires/>`` needs to be populated. Within, we can specify such
+There are two ways to determine which distro will be installed.  You can
+use ``<distroRequires/>`` node to let the scheduler pick a distro that has been
+imported into Beaker.  Or, you can specify your own distro via the 
+``<distro/>`` node.  You have to specify one of them.
+
+Within the ``<distroRequires/>``, we can specify such
 things as as ``<distro_arch/>``, ``<distro_name/>`` and
 ``<distro_method/>``. This relates to the Distro architecture, the name
 of the Distro, and it's install method (i.e nfs,ftp etc) respectively.
@@ -202,6 +206,78 @@ we do want that value, ``!=`` means we do not want that value.
         </recipeSet>
     </job>
 
+Within the ``<distro/>``, you must specify the full url in ``<tree/>``.
+``<kernel/>``, ``<initrd/>`` and ``<image/>`` should all be a relative
+url attribute based on ``<tree/>`` url attribute. ``<arch/>`` and ``<osversion/>``
+must be present as well.  ``<image/>`` referenced above is optional but allows you
+to use a custom bootloader for network installs. This option requires the dhcp
+server to be configured correctly.
+
+::
+
+    <job group='product-QA'>
+      <whiteboard>
+        Apache 2.2 test
+      </whiteboard>
+        <recipeSet>
+          <recipe kernel_options="" kernel_options_post="" ks_meta="" role="None" whiteboard="Lab Controller">
+            <packages>
+              <package name="emacs"/>
+              <package name="vim-enhanced"/>
+              <package name="unifdef"/>
+              <package name="mysql-server"/>
+              <package name="MySQL-python"/>
+              <package name="python-twill"/>
+            </packages>
+
+            <repos>
+              <repo name="myrepo_1" url="http://my-repo.com/tools/beaker/devel/"/>
+            </repos>
+            <distro>
+              <tree url="http://my-server.example.com/distros/MyCustomLinux1.0/Server/aarch64/os/"/>
+              <kernel url="pxeboot/vmlinuz"/>
+              <initrd url="pxeboot/initrd"/>
+              <image url="EFI/BOOT/grubaa64.efi"/>
+              <arch value="aarch64"/>
+              <osversion major="RedHatEnterpriseLinux8"/>
+            </distro>
+          </recipe>
+        </recipeSet>
+    </job>
+
+In order for the optional image loader to work the dhcp server must be configured
+properly.  Here is an example dnsmasq.conf which specifies the image in the correct
+path for the Systems under test.
+
+::
+
+    except-interface=lo
+    bind-dynamic
+    listen-address=10.60.0.190
+
+    resolv-file=/etc/resolv.conf
+    dhcp-range=10.60.0.20,10.60.0.100
+    dhcp-ignore=tag:!known
+    dhcp-option=option:router,10.60.0.190
+
+    # Use this server as a time and name server for machines in the lab
+    dhcp-option=option:dns-server,10.60.0.190
+    dhcp-option=option:ntp-server,192.168.1.3
+    domain=example.local
+    tftp-root=/var/lib/tftpboot
+    dhcp-boot=pxelinux.0
+
+    # Use a combination of /etc/ethers & /etc/hosts instead of this ?
+    dhcp-boot=tag:sut1_boot,bootloader/sut1.example.local/image,10.60.0.190
+    dhcp-boot=tag:sut2_boot,bootloader/sut2.example.local/image,10.60.0.190
+
+    dhcp-host=52:54:00:EF:C0:2C,sut1,10.60.0.51,set:sut1_boot
+    address=/sut1.example.local/10.60.0.51
+    dhcp-host=52:54:00:EF:C0:2D,sut2,10.60.0.52,set:sut2_boot
+    address=/sut2.example.local/10.60.0.52
+
+    # vim: set ft=dnsmasq:
+
 .. _host-requires:
 
 ``<hostRequires/>`` has similar attributes to ``<distroRequires/>``