README: update

@jeffh I'd like to remove most of the info on Nimble from the Quick README, and instead point readers to Nimble's README (I think it'll be easier to keep everything up to date that way). In preparation for doing so, I thought I'd make the two a bit more consistent:

- Use 1st-person tense
- Add more explanations on why I would use Nimble instead of XCTest assertions
- Make it clear whether the language is Swift or Objective-C
- (Nitpick) Use `#` instead of `=` to mark headers

I wasn't able to finish tonight, but here's the work-in-progress in case you have any thoughts/feedback. Also let me know if you'd rather leave the README as-is--that's certainly an option. :v: :sparkles: