This is the most lightweight way to add the task - no additional task-running software required. First:

$ npm install jsdoc-to-markdown --save-dev

Then, in the "scripts" section of package.json, add a docs task. For example:

{
  "scripts": {
    "docs": "jsdoc2md lib/*.js > api.md"
  }
}

Now, project documentation is generated like so:

$ npm run docs