clarify that "build & test everything" Grunt task requires Jekyll

Refs #13131.

docs/_includes/getting-started/grunt.html

@@ -28,7 +28,7 @@
   <p>Builds and tests CSS, JavaScript, and other assets which are used when running the documentation locally via <code>jekyll serve</code>.</p>
 
   <h3><code>grunt</code> (Build absolutely everything and run tests)</h3>
-  <p>Compiles and minifies CSS and JavaScript, builds the documentation website, runs the HTML5 validator against the docs, regenerates the Customizer assets, and more. Usually only necessary if you're hacking on Bootstrap itself.</p>
+  <p>Compiles and minifies CSS and JavaScript, builds the documentation website, runs the HTML5 validator against the docs, regenerates the Customizer assets, and more. Requires <a href="http://jekyllrb.com/docs/installation/">Jekyll</a>. Usually only necessary if you're hacking on Bootstrap itself.</p>
 
   <h2 id="grunt-troubleshooting">Troubleshooting</h2>
   <p>Should you encounter problems with installing dependencies or running Grunt commands, first delete the <code>/node_modules/</code> directory generated by npm. Then, rerun <code>npm install</code>.</p>