Slider interface is not documented in slider package README

[ranile]

Unlike other components (like mwc-list), Slider interface mentioned in slider component's Event section is undocumented.

[e111077]

Hello, slider is undergoing a complete rewrite for #523, so the documentation will not be updated as this documentation will be obviated by that issue, but Slider refers to the instance of the actual element itself. i.e. <mwc-slider> or this