docs: Add Host sFlow into monitoring apps

Reported-by: NPeter Phaal <peter.phaal@gmail.com>
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/apps.html.in

@@ -321,6 +321,12 @@
         For a full description, please refer to the libvirt section in the
         collectd.conf(5) manual page.
       </dd>
+      <dt><a href="http://host-sflow.sourceforge.net/">Host sFlow</a></dt>
+      <dd>
+        Host sFlow is a lightweight agent running on KVM hypervisors that
+        links to libvirt library and exports standardized cpu, memory, network
+        and disk metrics for all virtual machines.
+      </dd>
       <dt><a href="http://honk.sigxcpu.org/projects/libvirt/#munin">Munin</a></dt>
       <dd>
         The plugins provided by Guido G&uuml;nther allow to monitor various things