Establish a convention for extending the styles object at the runtime

`react-css-modules` [uses `styles` property to overwrite the default component styles](](https://github.com/gajus/react-css-modules#extending-component-styles)).

`generateClassName` could look for `props.styles` and `this.props.styles` when constructing the `className` value to determine whether a style needs to be overwritten.

There could be an option to force use of `generateClassName` even when `styleName` is a string literal value.

Start by reading the runtime handling around generateClassName and the documented styles extension behavior in the issue's react-css-modules link. Clarify whether props.styles, this.props.styles, and the proposed option are required, then define the expected className behavior and document the resulting convention.

Written by the indexing model from the issue text.