[spp] [PATCH 15/15] docs: add how to develop guide for SPP container

[Nakamura Hioryuki]

> +How to Define Your App Launcher
> +===============================
> +
> +SPP container is a set of python script for launching DPDK application
> +on a container with docker command. You can launch your own application
> +by preparing a container image and intall your application in
> +the container.

s/intall/install/