gpio: Update README.md and add one for gpio

This updates the main README and adds a specific one for GPIO crate.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

README.md

@@ -8,6 +8,7 @@ crates.
 
 Here is the list of device backends that we support:
 
+- [GPIO](https://github.com/rust-vmm/vhost-device/blob/main/gpio/README.md)
 - [I2C](https://github.com/rust-vmm/vhost-device/blob/main/i2c/README.md)
 - [RNG](https://github.com/rust-vmm/vhost-device/blob/main/rng/README.md)
 

gpio/README.md

@@ -1 +1,65 @@
 # vhost-device-gpio - GPIO emulation backend daemon
+
+## Description
+This program is a vhost-user backend that emulates a VirtIO GPIO device. This
+program takes a list of gpio devices on the host OS and then talks to them via
+the /dev/gpiochip{X} interface when a request comes from the guest OS for an
+GPIO device.
+
+This program is tested with QEMU's `-device vhost-user-gpio-pci` but should
+work with any virtual machine monitor (VMM) that supports vhost-user. See the
+Examples section below.
+
+## Synopsis
+
+**vhost-device-gpio** [*OPTIONS*]
+
+## Options
+
+.. program:: vhost-device-gpio
+
+.. option:: -h, --help
+
+  Print help.
+
+.. option:: -s, --socket-path=PATH
+
+  Location of vhost-user Unix domain sockets, this path will be suffixed with
+  0,1,2..socket_count-1.
+
+.. option:: -c, --socket-count=INT
+
+  Number of guests (sockets) to attach to, default set to 1.
+
+.. option:: -l, --device-list=GPIO-DEVICES
+
+  GPIO device list at the host OS in the format:
+      <device1>[:<device2>]
+
+      Example: --device-list "2:4:7"
+
+  Here, each GPIO devices correspond to a separate guest instance, i.e. the
+  number of devices in the device-list must match the number of sockets in the
+  --socket-count. For example, the GPIO device 0 will be allocated to the guest
+  with "<socket-path>0" path.
+
+## Examples
+
+The daemon should be started first:
+
+::
+
+  host# vhost-device-gpio --socket-path=gpio.sock --socket-count=1 --device-list 0:3
+
+The QEMU invocation needs to create a chardev socket the device can
+use to communicate as well as share the guests memory over a memfd.
+
+::
+
+  host# qemu-system \
+      -chardev socket,path=vgpio.sock,id=vgpio \
+      -device vhost-user-gpio-pci,chardev=vgpio,id=gpio \
+      -m 4096 \
+      -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on \
+      -numa node,memdev=mem \
+      ...