New policy files for Chef Client.

CHANGELOG.md: One line description of the changes.
RELEASE_NOTES.md: Important changes and breaking changes that needs to be called out in more detail in Release Notes.
DOC_CHANGES.md: Required doc changes in chef-docs.
CONTRIBUTIONS.md: One line description of the contributions with contributor info.

@danielsdeleo / @lamont-granquist / @adamedx thoughts? 

@btm I recall we wanted to add the MVPs into here as well. Feel free to push to this branch if that's ready to go :)