Updated contribution policies

[Barret Rhoden]

I updated our contribution policies (see origin/master).

The biggest thing is that all developers must agree to the Developer
Certificate of Origin (attached and available at
http://developercertificate.org/). If you do not agree to this for any
existing code, speak up and I'll remove the offending code. This has
always been our policy, in spirit, but not in writing.

For people interested in adding code to Akaros, there's a lot of info
in Documentation/Contributing, including a bunch of commands from my
git workflow.

Enjoy.

Barret