Update instructions for building docs locally

docs/README.md

@@ -22,17 +22,46 @@ eventually closed, so don't do that.
 
 How can I build these docs manually?
 ------------------------------------
-In the [main Select2 repository][select2-source], you can build the
-documentation by executing
+
+### Requirements
+
+##### Ruby and Jekyll
+
+Jekyll is a static site builder written in Ruby.  You will need **the latest version** of [Jekyll][jekyll] installed to build the Select2 documentation.
+
+First, [make sure that you have a Ruby environment set up][jekyll-quickstart].  Then, to install Jekyll:
+
+`gem install jekyll bundler`
+
+To upgrade Jekyll:
+
+`gem update jekyll`
+
+##### npm and Grunt
+
+Select2 uses [Grunt][grunt], which is an npm package, as a task runner. To run the build task, you will need to first [install the latest version of npm and Node.js][nodejs] as well as the [Grunt CLI][grunt-cli].
+
+### Building the docs with Grunt
+
+This assumes that you have already cloned the Select2 repo to your local development environment.  In the [main Select2 repository][select2-source], you will first need to install the npm dependencies locally:
+
+```
+npm install
+```
+
+Then, you can build and launch the documentation by running
 
 ```bash
 grunt docs
 ```
 
-Which will start up the documentation on port 4000. You will need
-[Jekyll][jekyll] installed to build the documentation.
+This will start up the documentation on port 4000 ([http://localhost:4000](http://localhost:4000)). 
 
 [jekyll]: http://jekyllrb.com/
+[jekyll-quickstart]: https://jekyllrb.com/docs/quickstart/
+[grunt]: https://gruntjs.com
+[grunt-cli]: https://gruntjs.com/getting-started
+[nodejs]: https://nodejs.org/en/download/
 [select2]: https://select2.github.io
 [select2-docs-source]: https://github.com/select2/select2.github.io
 [select2-source]: https://github.com/select2/select2