Finalizing our source code policy

This takes @rjmajma's work on his fork, moves it into an 18F branch, and clears up some mis-synced commits. The best way to view this PR is by using GitHub's prose rendering, here:

[replacing link once PR is created, won't show up in your email]

**Let's use this PR to finalize the policy.** Let's discuss proposed changes here, and as they are agreed to, anyone with write privileges on this repo can push more commits to the `raph-finalizing` branch, which will automatically update this PR.

When this PR is merged, let's consider the source code policy ready for blogging-about, and ready for linking-to. We should be proud of it. But let's not consider it "done" forever -- further issues and PRs should be welcome, from both inside and outside 18F.