Better documentation for developers extending this package

[mikemonteith]

The documentation in the service manual is excellent for people who want to use the nhsuk-library components in their project, but there is a lack of documenatation for developers who are creating new components that they intend to contribute back into the service manual, or creating custom components specific to their service.

Things that could be documented better:

• Sass variables
• Sass mixins
• CSS Utility classes
• Best practices for creating your own good CSS

[mikemonteith]

GOV have this https://frontend.design-system.service.gov.uk/sass-api-reference/

[mikemonteith]

Also could document how to include the variables and mixins without also including any actual CSS styles.
This is important for teams who are using a framework which compiles scss such as sass-loader for webpack.