readme: add a note about using virtiofs as rootfs.

This is convenient for development.

BUG=None
TEST=None

Change-Id: I2ee06d70a7e47ea1c4de0e9cb54d382cb8b050d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2460862
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Commit-Queue: Dylan Reid <dgreid@chromium.org>
Commit-Queue: Lepton Wu <lepton@chromium.org>
Tested-by: Lepton Wu <lepton@chromium.org>
Auto-Submit: Lepton Wu <lepton@chromium.org>

README.md

@@ -89,6 +89,8 @@ build directory in the case of x86 at `arch/x86/boot/compressed/vmlinux`.
 
 ### Rootfs
 
+#### With a disk image
+
 In most cases, you will want to give the VM a virtual block device to use as a
 root file system:
 
@@ -113,6 +115,16 @@ crosvm run --rwdisk "${ROOT_IMAGE}" -p "root=/dev/vda" vmlinux
 >**NOTE:** If more disks arguments are added prior to the desired rootfs image,
 the `root=/dev/vda` must be adjusted to the appropriate letter.
 
+#### With virtiofs
+
+Linux kernel 5.4+ is required for using virtiofs. This is convenient for testing.
+The file system must be named "mtd*" or "ubi*".
+
+```bash
+crosvm run --shared-dir "/:mtdfake:type=fs:cache=always" \
+    -p "rootfstype=virtiofs root=mtdfake" vmlinux
+```
+
 ### Control Socket
 
 If the control socket was enabled with `-s`, the main process can be controlled