Add user guide to README.md

[eecavanna]

I see that this repository contains files, but I don't understand how I can use them. I suggest adding a "Usage" or "User Guide" section to README.md that includes instructions someone can follow to do something using the contents of this repo.

For example:

Usage

1. Identify the thing you want to make by looking at the targets defined in Makefile
2. Run $ make <target>
3. View the result in <directory>