Clarify resolve tag + macro docs, add CRA section

[karlhorky]

Hey @giuseppeg, thanks for this library!

As we discussed, here is a pull request that cleans up the docs around the resolve tag, Babel macros and adds a create-react-app section. More specifically, it:

• Clarifies the use case of the resolve tag
• Removes the extra, unrelated <style jsx> line from the resolve tag example
• Makes a note of how the dynamic styles are supported (via template string interpolation)
• Fixes some casing of Babel
• Add some guidance / explainer text before the Babel macros installation instructions
• Add a full Babel macros example
• Add a section dedicated to create-react-app, with link to the project

If any of these things are questionable or you have some feedback, would be glad to discuss further!