doc: add documentaion for rbd-nbd mounter

Closes #2124

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>

README.md

@@ -59,6 +59,9 @@ NOTE:
 
 ### Ceph-CSI features and available versions
 
+Please refer [rbd nbd mounter](./docs/rbd-nbd.md#support-matrix)
+for its support details.
+
 | Plugin | Features                                                  | Feature Status | CSI Driver Version | CSI Spec Version | Ceph Cluster Version | Kubernetes Version |
 | ------ | --------------------------------------------------------- | -------------- | ------------------ | ---------------- | -------------------- | ------------------ |
 | RBD    | Dynamically provision, de-provision Block mode RWO volume | GA             | >= v1.0.0          | >= v1.0.0        | Nautilus (>=14.0.0)  | >= v1.14.0         |

docs/rbd-nbd.md

@@ -0,0 +1,48 @@
+# RBD NBD Mounter
+
+- [RBD NBD Mounter](#rbd-nbd-mounter)
+  - [Overview](#overview)
+  - [Configuration](#configuration)
+  - [Status](#status)
+  - [Support Matrix](#support-matrix)
+    - [CSI spec and Kubernetes version compatibility](#csi-spec-and-kubernetes-version-compatibility)
+
+## Overview
+
+The RBD CSI plugin will provision new RBD images and attach and mount those
+to workloads. Currently, the default mounter is krbd, which uses the kernel
+rbd driver to mount the rbd images onto the application node. Here on
+at ceph-csi we will also have a userspace way of mounting the RBD images,
+via RBD-NBD.
+
+[Rbd-nbd](https://docs.ceph.com/en/latest/man/8/rbd-nbd/) is a client for
+RADOS block device (rbd) images like the existing rbd kernel module. It
+will map an rbd image to an NBD (Network Block Device) device, allowing
+access to it as a regular local block device.
+
+It’s worth to make a note that the rbd-nbd processes will run on the
+client-side, which is inside the `csi-rbdplugin` node plugin.
+
+## Configuration
+
+To use the rbd-nbd mounter for RBD-backed PVs, set `mounter` to `rbd-nbd`
+in the StorageClass.
+
+## Status
+
+Rbd-nbd support status: **Alpha**
+
+## Support Matrix
+
+| Features                                 | Feature Status | CSI Driver Version | Ceph Cluster Version | CSI Spec Version | Kubernetes Version |
+| ---------------------------------------- | -------------- | ------------------ | -------------------- | ---------------- | ------------------ |
+| Creating and deleting snapshot           | Alpha          | >= v3.4.0          | Pacific (>=16.0.0)   | >= v1.0.0        | >= v1.17.0         |
+| Creating and deleting clones             | Alpha          | >= v3.4.0          | Pacific (>=16.0.0)   | >= v1.0.0        | >= v1.17.0         |
+
+`NOTE`: The `Alpha` status reflects possible non-backward compatible
+changes in the future, and is thus not recommended for production use.
+
+### CSI spec and Kubernetes version compatibility
+
+Please refer to the [matrix](https://kubernetes-csi.github.io/docs/#kubernetes-releases)
+in the Kubernetes documentation.