Add paragraph to team practices doc encouraging upstream contributions

This adds a section explicitly encouraging teammates to take any improvements they make to outside open source projects in the course of their work and contribute them upstream.

It does what I think is a competent but not totally rigorous job of explaining why this is okay, and how government works interact with outside open source projects' contributing agreements.

I'd love @benbalter's :eyes: on this one, along with @rjmajma @leah and @yuda.
@benbalter by "outside" I didn't mean "outside government", I meant "outside 18F", to differentiate it from our own projects.

And "are encouraged" is better. Revising now.