Add language about CFPB contribs. to a joint work

Making it clear that the work performed by us is in the Public Domain.
Add suggested CHANGELOG format

Based on recommendations from http://keepachangelog.com/
Use official CC0 plain text legal code

As found here:
http://creativecommons.org/publicdomain/zero/1.0/legalcode.txt
Merge pull request #5 from Scotchester/master

Improve joint work language; add suggested CHANGELOG format