docs/rpmsgfs: initial docs on RPMsg file system usage.

This patch adds steps about configuring and using the RPMsg file system
with simple example.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

Documentation/components/filesystem/rpmsgfs.rst

@@ -2,4 +2,31 @@
 RPMSG File System
 =================
 
-Use rpmsg file system to mount remote directories to local.
+Once RPMsg link is available, we can use rpmsg file system to mount remote directories with the help of RPMsg file system drivers.
+
+Here we show an example of mounting and using a master file system path from remote side, it is as easy as using a local file system.
+
+Building
+========
+
+At file system server side (the master), we need enable the ``CONFIG_FS_RPMSGFS_SERVER`` configuration. 
+
+At file system client side (the remote), we need enable the ``CONFIG_FS_RPMSGFS`` configuration.
+
+Then we build the two sides accordingly.
+
+Running
+=======
+
+Using the following command to mount the master's ``/proc`` file system to ``/proc.master`` from the ``nsh`` sessino of the remote node.
+
+.. code:: console
+
+  remote> mount -t rpmsgfs -o cpu=master,fs=/proc /proc.master
+  remote> cat /proc/uptime /proc.master/uptime 
+        39.06                                                                      
+        39.06                                                                      
+  remote>
+
+Note the ``-o cpu=master,fs=/proc`` specifies the ``master`` node's ``/proc`` path as the source, the ``/proc.master`` is the mount point at remote side. All files under that mount point is actually hosted at the master side. The ``-t rpmsgfs`` selects the RPMsg file system driver to serve the operation.
+