Add Varnish Docs

- Fix #1808

doc/Extensions/Varnish.md

@@ -0,0 +1,53 @@
+# Setting up Varnish 
+
+This document will explain how to setup Varnish for LibreNMS.
+
+### Varnish installation
+This example is based on a fresh LibreNMS install, on a minimimal CentOS installation.
+In this example, we'll use the default package available through yum.
+
+- Install Varnish
+
+```ssh
+yum install varnish
+chkconfig varnish on
+```
+- Confirm that Varnish has been installed 
+
+```ssh
+varnishd -V
+varnishd (varnish-2.1.5 SVN )
+```
+- Change the webservers port to 8080, since we'll put Varnish in front(Or whatever you prefer) 
+
+- Point Varnish towards the webserver by editing the default.vcl 
+
+```ssh
+vi /etc/varnish/default.vcl
+
+backend default {
+  .host = "127.0.0.1";
+  .port = "8080";
+}
+
+```
+- Change the default port Varnish listens on
+
+```ssh
+vi /etc/sysconfig/varnish
+
+VARNISH_LISTEN_PORT=80
+```
+
+- Restart webserver(Apache in this case) and start Varnish afterwards
+
+```ssh
+service httpd restart
+service varnish start
+```
+
+- Browse around the webui to build up the cache, verify that the cache is working afterwards
+
+```ssh
+varnishstat
+```