Quick

CONTRIBUTING: Use .xcworkspace

- Advise contributors to use workspace file.
- Remove "BDD"--it's a loaded term. You can use Quick to write
  non-behavioral tests, just as you can use XCTest to write behavioral
  tests. I believe it's often used to denote "xSpec"-style frameworks,
  but I'd prefer something less ambiguous.
- Remove notes on adding files to both iOS and OS X targets. CI script
  now tests both iOS and OS X builds, so it's unlikely an error like
  this will make it into the build.