NAS Backup and Recovery plugin usage (#439)

Adding NAS Backup and Recovery plugin usage.

source/adminguide/nas_plugin.rst

@@ -56,6 +56,33 @@ admins can create new backup offerings by selecting the zone and the backup
 repository. These backup offerings are then assigned and used with KVM instances
 to perform support B&R actions and operations.
 
+Using the NAS Backup and Recovery Plugin
+----------------------------------------
+To use the NAS Backup and Recovery Plugin, the Backup and Recovery framework needs to be enabled first. Then the backup plugin 'nas' needs to be enabled on either the global setting or on the zone settings. 
+
+================================= ========================
+Configuration                     Value
+================================= ========================
+backup.framework.enabled          true
+backup.framework.provider.plugin  nas
+================================= ========================
+
+Once the above two configurations are set, restart the cloudstack-management service. Once the service is restarted we can add the backup repository for the 'nas' Backup and Recovery plugin.
+Navigate to the configuration -> Backup Repository. Click on 'Add Backup Repository' and fill the form. 
+
+=================== ========================
+Field               Value
+=================== ========================
+Name                N suitable name to represent the Backup Repository
+Address             URL, in case of NFS <server IP>:/path
+Type                NFS ( only NFS type in 4.20)
+label.mountopts     Any mount point options to be passed while mouting this storage on the hypervisor. 
+Zone                The zone in CloudStack with which this Backup Repository must be associated.
+=================== ========================
+
+
+Once the Backup Repository is created, we need to add a Backup Offering, in this plugin the Backup offering is a placeholder to associate an instance to a Backup Repository. While creating the Backup Offering, select the desired Backup Repository. Associate the Backup Offering on an instance to create an Adhoc or scheduled backup. 
+
 Support Information and Limitation
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