Added documentation to enable billing module

doc/Billing_Module.md

@@ -0,0 +1,10 @@
+To enable and use the billing module you need to perform the following steps:
+
+Edit config.php and add (or enable) the following line near the end of the config
+   $config['enable_billing'] = 1; # Enable Billing
+
+Edit /etc/cron.d/librenms and add the following:
+  */5 * * * * root /opt/librenms/poll-billing.php >> /dev/null 2>&1
+  01 * * * * root /opt/librenms/billing-calculate.php >> /dev/null 2>&1
+
+Create billing graphs as required.