Added documentation of slots

Slot

input in component

<template>
  <div>
    <!-- Use slot header -->
    <slot name="header"></slot>

    <!-- Use slot footer -->
    <slot name="footer"></slot>
  </div>
</template>

output in JSON

"slots": {
    "header": {
      "description": "Use this slot header"
    },
    "footer": {
      "description": "Use this slot footer"
    }
}