Reorganize README

[danburzo]

I'd like to attempt reorganizing the README and include more details on the following:

• The constructor options
• The shape of the object returned by Readability.parse()
• The Readability.isReaderable() function
• Usage in the browser vs. Node.js

I thought I'd open an issue to get confirmation before starting a PR.

[gijsk]

Sure, if we can make this clearer that would be great!