kernel: add usage in readme

Add usage of build-kernel.sh to the readme

Signed-off-by: Jimmy Xu <junming.xjm@antfin.com>

kernel/README.md

@@ -1,6 +1,7 @@
 # Build Kata Containers Kernel
 
 * [Requirements](#requirements)
+* [Usage](#usage)
 * [Setup kernel source code](#setup-kernel-source-code)
 * [Build the kernel](#build-the-kernel)
 * [Install the Kernel in the default path for Kata](#install-the-kernel-in-the-default-path-for-kata)
@@ -17,6 +18,54 @@ automates the process to build a kernel for Kata Containers.
 The `build-kernel.sh` script requires an installed Golang version matching the
 [component build requirements](https://github.com/kata-containers/documentation/blob/master/Developer-Guide.md#requirements-to-build-individual-components).
 
+## Usage
+
+```
+$ ./build-kernel.sh -h
+Overview:
+
+	Build a kernel for Kata Containers
+
+Description: This script is the *ONLY* to build a kernel for development.
+
+
+Usage:
+
+	build-kernel.sh [options] <command> <argument>
+
+Commands:
+
+- setup
+
+- build
+
+- install
+
+Options:
+
+	-c <path>   : Path to config file to build a the kernel.
+	-d          : Enable bash debug.
+	-e          : Enable experimental kernel.
+	-f          : Enable force generate config when setup.
+	-g <vendor> : GPU vendor, intel or nvidia.
+	-h          : Display this help.
+	-k <path>   : Path to kernel to build.
+	-p <path>   : Path to a directory with patches to apply to kernel.
+	-t          : Hypervisor_target.
+	-v          : Kernel version to use if kernel path not provided.
+```
+
+Example:
+```
+$ ./build-kernel.sh -v 4.19.86 -g nvidia -f -d setup
+```
+> **Note**
+> - `-v 4.19.86`: Specify the guest kernel version.
+> - `-g nvidia`: To build a guest kernel supporting Nvidia GPU.
+> - `-f`: The .config file is forced to be generated even if the kernel directory already exists.
+> - `-d`: Enable bash debug mode.
+
+
 ## Setup kernel source code
 
 ```bash

kernel/kata_config_version

@@ -1 +1 @@
-69
+70