doc: css styling guidelines

[mgenov]

A documentation with guidelines about postcss and styling need to be added in the project as separate section.

[mgenov]

Here are few methodologies that could be considered:

• BEM
  • https://en.bem.info/methodology/
  • Shown as example in: Style Your application from Start to Finish
  • Looks simple one
• SUITCSS
  • https://suitcss.github.io/
• AtomicCSS
  • https://acss.io/

Overall guide about styling in react: http://andrewhfarmer.com/how-to-style-react/