Add "CONTRIBUTING.md"

[AParise1217]

Most popular open source repositories have a "CONTRIBUTING.md" file that explains the guidelines for contributing back to the repo. I didn't see one here for fflib.

Here is a link that explains what this is: https://blog.github.com/2012-09-17-contributing-guidelines/

I'd like to contribute, but my company needs to know about the legal obligations they need to take care of.

[afawcett]

This is a good suggestion @AParise1217. As I no longer work for FinancialForce i cannot speak to this. Maybe its something @agarciaffdc can raise for further internal discussion?

[agarciaffdc]

@AParise1217 I really like the idea. I will back to you as soon as I have a proper answer to you.