Clarifies language on working in the open, adds team practices doc

**Update**: After extensive discussion in #16 and #18, this PR now clarifies the policy to clearly state that the policy covers working in the open, not just releasing source code at the conclusion of development.

It also adds a `practice.md` document with 18F team guidance and our current list of exceptions.

Original pull request decription follows.

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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").

There is a general section on how we treat project work before an inter-agency agreement (IAA) is signed.