message: docs: document cover key
diff: @@ -95,3 +95,18 @@ module.exports = {
  render: "./mount.js"
  };
  ```
  +
  +### cover
  +
  +An absolute or relative module id `string`, resolved relative to `patternplate.config.js`.
  +The exports of the resolved module are used as a specialized demo that is displayed independently
  +of the patternplate web interface on `/`
  +
  +**Examples**
  +
  +```js
  +module.exports = {
  + // use the file cover.js next to patternplate.config.js
  + render: "./cover.js"
  +};
  +```
  