docs: kernel: Add README.

Add README to explain how to build kernel

Signed-off-by: Jose Carlos Venegas Munoz <jose.carlos.venegas.munoz@intel.com>

kernel/README.md

@@ -0,0 +1,38 @@
+# Build Kata Containers Kernel
+
+This document explains the steps to build a compatible kernel with Kata
+Containers. To do this use build-kernel.sh, this script automates the
+process to build a kernel for Kata Containers. 
+
+## Setup kernel source code
+
+```bash
+./build-kernel.sh setup
+```
+
+The script `./build-kernel.sh` tries to apply the patches from
+`${GOPATH}/src/github.com/kata-containers/packaging/kernel/patches/` when it
+sets up a kernel. If you want to add a source modification, add a patch on this
+directory.
+
+The script also adds a kernel config file from
+`${GOPATH}/src/github.com/kata-containers/packaging/kernel/configs/` to .config
+in the kernel source code. You can modify it as needed.
+
+# Build the kernel
+
+After the kernel source code is ready it is possible to build the kernel.
+
+```bash
+./build-kernel.sh build
+```
+
+
+## Install the Kernel in the default path for Kata
+
+Kata Containers uses some default path to search a kernel to boot. To install
+on this path, the following command will install it to the default Kata
+containers path.
+```bash
+./build-kernel.sh install
+```