message: docs: add node.js version info into the universal guide
  Previously we didn't document what versions were supported. Since universal requires node.js I'm adding version
  information into the guide as a informative callout.
  Fixes
  PR Close
diff: @@ -13,6 +13,13 @@ For a more detailed look at different techniques and concepts surrounding SSR, c
  You can easily prepare an application for server-side rendering using the [Angular CLI](guide/glossary#cli).
  The CLI schematic `@nguniversal/express-engine` performs the required steps, as described below.
  +<div class="alert is-helpful">
  +
  + Angular Universal requires an [active LTS or maintenance LTS](https://nodejs.org/about/releases) version of Node.js.
  + See the `engines` property in the [package.json](https://unpkg.com/browse/@angular/platform-server/package.json) file to learn about the currently supported versions.
  +
  +</div>
  +
  <div class="alert is-helpful">
  **Note:** <live-example downloadOnly>Download the finished sample code</live-example>,
  