Add section to Guide/FAQ for "Do I even need to localize?"

[ericf]

I think it's important to stress that even if you're not deploying an app into multiple locales, FormatJS is still very much helpful and relevant for English or any single locale. <-- We should add that to the site somewhere prominent.

[juandopazo]

It sounds like something that should go in the homepage.

[caridy]

yeah, it outsource the formatting logic out of the view implementation...