capturing @rjmajma's changes in a smaller PR, inside 18F
reformat the pre-decisional header
Tidying up links, no need to repeat the same one in markdown footnotes
Fix up a sentence left mid-edit
Small language tweak

Thanking people for their work, not in it.
Adds CONTRIBUTING and LICENSE files.

Mentioned in the README as covering this repo.
Mentioned in the source code policy as our default files for our
projects. Rearranges licensing section of policy a bit.
Merge branch 'raph-finalizing' of github.com:18F/open-source-policy into raph-finalizing
Renames the policy file to just policy.md.

Since the rest of the information is baked into the URL.
Changes to Exceptions

Removing exceptions 3 & 4 and adding in language some language that dictates how we will respond if there is a compelling interest to not go open source.
sentence dropped off, was redundant anyhow
made the footnote a <sup>
fix word typo
18F: An Open Source Team
where
clarify domestic public domain
update licensing header
typo/grammar tweaks
adding emphasis for contractor use of open source
more grammar edits
removing gratuitous "gratuitous"