Add customStyles section to documentation

[tordans]

The page https://github.com/casesandberg/react-color/blob/master/docs/documentation/03.05-customStyles.md is ideal to explain how to customise styles but is missing from https://casesandberg.github.io/react-color/#api.

Adding this will also resolve the need to document the styles props individually at https://github.com/casesandberg/react-color/pull/816