Update prop chart to include prefixCls

[SebastianKurp]

Hey!
Noticed that prefixCls is a prop that is exposed, but not documented. So I added it to the props chart in the readMe.
Here's an example code snippet of it in use, with a screenshot of the HTML to show that it is passing in the prefixCls to the className.

  <Slider
    prefixCls="predixClsTest"
  />

[tlehtimaki]

This seems not to be the only one missing from the README.md. If you look at the src/Slider.tsx

slider/src/Slider.tsx

Lines 7 to 47 in b5a337e

	export interface SliderProps {
	value?: number;
	defaultValue?: number;
	min?: number;
	max?: number;
	step?: number;
	prefixCls?: string;
	onChange?: (value: number) => void;
	onBeforeChange?: (value: number) => void;
	onAfterChange?: (value: number) => void;
	vertical?: boolean;
	included?: boolean;
	disabled?: boolean;
	reverse?: boolean;
	minimumTrackStyle?: React.CSSProperties;
	trackStyle?: React.CSSProperties;
	handleStyle?: React.CSSProperties;
	tabIndex?: number;
	ariaLabelForHandle?: string;
	ariaLabelledByForHandle?: string;
	ariaValueTextFormatterForHandle?: string;
	startPoint?: number;
	handle: (props: {
	className: string;
	prefixCls?: string;
	vertical?: boolean;
	offset: number;
	value: number;
	dragging?: boolean;
	disabled?: boolean;
	min?: number;
	max?: number;
	reverse?: boolean;
	index: number;
	tabIndex?: number;
	ariaLabel: string;
	ariaLabelledBy: string;
	ariaValueTextFormatter: string;
	style?: React.CSSProperties;
	ref?: React.Ref<any>;
	}) => React.ReactElement;

The docs do not match with the interface very well or am I missing somehing? Maybe there should be a seperate issue and fix all at once.

And here it seems that the defaults set can be seen from src/common/createSlider.tsx

slider/src/common/createSlider.tsx

Lines 16 to 46 in b5a337e

	static defaultProps = {
	...Component.defaultProps,
	prefixCls: 'rc-slider',
	className: '',
	min: 0,
	max: 100,
	step: 1,
	marks: {},
	handle(props) {
	const { index, ...restProps } = props;
	delete restProps.dragging;
	if (restProps.value === null) {
	return null;
	}
	return <Handle {...restProps} key={index} />;
	},
	onBeforeChange: noop,
	onChange: noop,
	onAfterChange: noop,
	included: true,
	disabled: false,
	dots: false,
	vertical: false,
	reverse: false,
	trackStyle: [{}],
	handleStyle: [{}],
	railStyle: {},
	dotStyle: {},
	activeDotStyle: {},
	};

[yoyo837]

Rebase please.