Added a document about interoperability between Swift and C++

This document discusses the design and tradeoffs for interoperability
between Swift and C++.

This document was written with valuable contributions from
Doug Gregor <[email protected]>, John McCall <[email protected]>,
Chris Lattner <[email protected]>, Dave Abrahams <[email protected]>,
Tony Allevato <[email protected]>, Mike Forster <[email protected]>,
Dave MacLachlan <[email protected]>, and Josh L <[email protected]>.

Author: gribozavr