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

Include additional contents
(adapted from https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md)

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>
Merge pull request #37 from gmacario/add-contributing

Add file CONTRIBUTING.md