Add code style documentation

[czaas]

This should be a generic JS SCSS overview of best practices as well as

• Component design: https://bitbucket.org/atlassian/atlaskit-mk-2/src/master/docs/guides/01-component-design.md
• Prop naming: https://bitbucket.org/atlassian/atlaskit-mk-2/src/master/docs/guides/02-naming-props.md
• Controlled vs uncontrolled https://bitbucket.org/atlassian/atlaskit-mk-2/src/master/docs/guides/10-controlled-uncontrolled-props.md