devices: vhost: user: Add README

BUG=none
TEST=none

Change-Id: I6a7b27c5dbb44d417ff9e0e7b213403efee92ec5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3320905
Auto-Submit: Keiichi Watanabe <keiichiw@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Commit-Queue: Chirantan Ekbote <chirantan@chromium.org>

devices/src/virtio/vhost/user/README.md

@@ -0,0 +1,34 @@
+# Vhost-user devices
+
+This directory contains the implementation of [vhost-user] devices and
+[virtio vhost-user] devices.
+
+## Code Locations
+
+*   [`vmm`](./vmm/) - Implements vhost-user vmm device; i.e. vhost-user master.
+*   [`device`](./device/) - Implements vhost-user device backend; i.e.
+    vhost-user slave.
+*   [`proxy.rs`](./proxy.rs) - Implements [virtio vhost-user] device, which
+    works like a proxy forwarding vhost-user messages to the guest via
+    virtqueues.
+
+## Usage
+
+### Vhost-user
+
+First, start a vhost-user device with the `crosvm device` command. Here we use
+the block device as an example, but the basic usage is same for all of devices.
+
+```bash
+$ crosvm device block --socket /path/to/socket --file /path/to/block.img
+```
+
+Then start a VM with a vhost-user block device by specifying the same socket
+path .
+
+```bash
+$ crosvm run -r rootfs.img --vhost-user-blk /path/to/socket <crosvm arguments>
+```
+
+[vhost-user]: https://qemu.readthedocs.io/en/latest/interop/vhost-user.html
+[virtio vhost-user]: https://wiki.qemu.org/Features/VirtioVhostUser