Add reference to `build-yocto` subproject

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>
Merge pull request #36 from gmacario/improve-readme

Add reference to `build-yocto` subproject
Create CONTRIBUTING.md
CONTRIBUTING.md: Start from a well-written example

Import contents from
https://github.com/thoughtbot/factory_girl_rails/blob/master/CONTRIBUTING.md
CONTRIBUTING.md: Customize for project easy-build

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>