Add optional cleanup task for old bill_data

doc/Extensions/Billing-Module.md

@@ -16,3 +16,17 @@ Edit `/etc/cron.d/librenms` and add the following:
 ```
 
 Create billing graphs as required.
+
+## Options
+
+Billing data is stored in the MySQL database, and you may wish to purge the detailed 
+stats for old data (per-month totals will always be kept).  To enable this, add the 
+following to `config.php`:
+
+```php
+$config['billing_data_purge'] = 12;     // Number of months to retain
+```
+
+Data for the last complete billing cycle will always be retained - only data older than
+this by the configured number of months will be removed.  This task is performed in the
+daily cleanup tasks.