doc: add design to support nfs in external cluster

with recent discussion we come to conclusion
nfs can be suported easily in the external cluster
so adding the design that we discussed

closes: rook#13277

Signed-off-by: parth-gr <partharora1010@gmail.com>

Documentation/CRDs/Cluster/external-cluster.md

@@ -214,6 +214,46 @@ Create the object store resources:
 If encryption or compression on the wire is needed, specify the `--v2-port-enable` flag.
 If the v2 address type is present in the `ceph quorum_status`, then the output of 'ceph mon data' i.e, `ROOK_EXTERNAL_CEPH_MON_DATA` will use the v2 port(`3300`).
 
+### NFS storage
+
+Make use of the NFS client running on the external ceph standalone cluster.
+
+Create the export,
+```console
+ceph nfs export create cephfs <nfs-client-name> /test <filesystem-name>
+```
+
+Create the PV,
+```console
+apiVersion: v1
+kind: PersistentVolume
+metadata:
+  name: <pv-name>
+spec:
+  capacity:
+    storage: <storage>
+  accessModes:
+  - ReadWriteOnce
+  nfs:
+    path: /test
+    server: <nfs-client-server-ip>
+  persistentVolumeReclaimPolicy: Retain
+```
+And PVC,
+```
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: nfs-claim1
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: <storage>
+  volumeName: <pv-name>
+```
+
 ## Exporting Rook to another cluster
 
 If you have multiple K8s clusters running, and want to use the local `rook-ceph` cluster as the central storage,