Add section about component styling

[dtslvr]

Hey guys,

It would be great to have a section about the best practices in component styling: Topics could be like:

• Using css / sass
• Using styleUrls
• Scoping (:host)
• How to use or import variables in css / sass

What do you think?

[d3lm]

Yep! Would you want to contribute some of those sections?