Update README.md

Add the introduction of how to mount and unmount in semu.

Author: MikazukiHikari

README.md

@@ -85,6 +85,26 @@ You can exit the emulator using: \<Ctrl-a x\>. (press Ctrl+A, leave it, afterwar
 * `initrd-image` is optional, as it specifies the user-specified initial RAM disk image.
 * `disk-image` is optional, as it specifies the path of a disk image in ext4 file system for the virtio-blk device.
 * `mount-folder` is optional, as it specifies the path of a folder you want to mount in host.
+
+## Mount and unmount a folder in semu
+
+To mount the folder in semu:
+
+```shell
+$ mount -t virtiofs myfs [mount-folder]
+```
+
+* `mount-folder` is the path of a folder you want to mount in semu.
+
+To unmount the folder in semu:
+
+```shell
+$ umount [mount-folder]
+```
+
+* `mount-folder` is the path of a folder you want to unmount in semu.
+
+
 ## Build Linux kernel image and root file system
 
 An automated build script is provided to compile the RISC-V cross-compiler, Busybox, and Linux kernel from source.