Document how to change styles for Twitter-Picker

[tordans]

This clarifies how to customise the styles.

It took me a while to figure out I need to add default to the styles hash. The styles example from https://casesandberg.github.io/react-color/#create-helpers EditableInput does not use the default hash. And the example at https://github.com/casesandberg/react-color/blob/master/docs/documentation/03.05-customStyles.md is not rendered at https://casesandberg.github.io/react-color/.