[docs] Explain how to pass props down to overridden components

[manuelkiessling]

No description provided.

[oliviertassinari]

@manuelkiessling I think that I understand the core issue behind this pull-request. Maybe we should be updating the demo to follow this pattern? However, I have two concerns here:

• <Button className={props.classes.button} {...props}> No, it's not correct. It should be:

const { classes, ...other } = props;

<Button className={classes.root} {...other}>

• If the override focus on style only, the simplest thing to do is:

import React from 'react';
import { withStyles } from '@material-ui/core/styles';
import Button from '@material-ui/core/Button';

export default withStyles({
  root: {
    background: 'linear-gradient(45deg, #FE6B8B 30%, #FF8E53 90%)',
    borderRadius: 3,
    border: 0,
    color: 'white',
    height: 48,
    padding: '0 30px',
    boxShadow: '0 3px 5px 2px rgba(255, 105, 135, .3)',
  },
})(Button);

[oliviertassinari]

@manuelkiessling Thank you for raising this concern :).