Adding an exceptions page, and a team FAQ page.

This adds a team FAQ page, with answers optimized for members of the (rapidly growing!) 18F family.

It also adds an **`exceptions.md`** file, which will be where we make good on the policy's [promise to publicly document](https://github.com/18F/open-source-policy/blob/master/policy.md#exceptions) exceptions (which are supposed to be "rare" and for a "compelling interest").

This is **not merge-ready** yet, as we're not done auditing for any exceptions we may need to list.

There is a general section on how we treat project work before an inter-agency agreement (IAA) is signed.