Playgrounds as Interactive Documents

This PR contains a blog post and playground showing how to use a playground as an interactive document for presentations or general reading/instruction.
`directions` -> did you mean `directories` or `folders`? 

`The second set ... is executed .. ` -> `The .swift files hold the code that will be executed in the Playground.`, a more direct sentence structure could be good for this paragraph.

`Last, the third` - only need one of these phrases.