Added Services extension doc to help people set this up

doc/Extensions/Services.md

@@ -0,0 +1,50 @@
+# Setting up Services
+
+Services within LibreNMS provides the ability to use Nagios plugins to perform additional monitoring outside of SNMP.
+
+These services are tied into an existing device so you need at least one device that supports SNMP to be able to add it 
+to LibreNMS - localhot is a good one.
+
+## Setup
+
+Firstly, install Nagios plugins however you would like, this could be via yum, apt-get or direct from source.
+
+Next, you need to enable the services within config.php with the following:
+
+```php
+$config['show_services']           = 1;
+```
+This will enable a new service menu within your navbar.
+
+```php
+$config['nagios_plugins']   = "/usr/lib/nagios/plugins";
+```
+
+This will point LibreNMS at the location of the nagios plugins - please ensure that any plugins you use are set to executable.
+
+Finally, you now need to add check-services.php to the current cron file (/etc/cron.d/librenms typically) like:
+```bash
+*/5 * * * * librenms /opt/librenms/check-services.php >> /dev/null 2>&1
+```
+
+Now you can add services via the main Services link in the navbar, or via the Services link within the device page.
+
+> **Please note that at present the service checks will only return the status and the response from the check 
+no graphs will be generated. **
+
+## Supported checks
+
+- ftp
+- icmp
+- spop
+- ssh
+- ssl_cert
+- http
+- domain_expire
+- mysql
+- imap
+- dns
+- telnet
+- smtp
+- pop
+- simap