Update SCSS Section

[lindseywild]

This came out of an HTML & CSS Task Force meeting.

• Rename SCSS to Styles (or something of the like)
• Anything regarding variables/variable naming, mixins, extend vs include
• Include references of how to compile Sass? It seems that this section assumes someone has used a CSS Preprocessor before -- or point someone in the right direction 🤷‍♀
• Any other updates you'd like to make!

This is a good reference! https://github.com/airbnb/css