Add an introduction and a prerequisite section. (#656)

resolves #655

Author: ahus1

CONTRIBUTING.adoc

@@ -3,19 +3,33 @@
 :uri-atom-textmate-language-raw: https://raw.githubusercontent.com/asciidoctor/atom-language-asciidoc/master/grammars/language-asciidoc.cson
 :uri-npm-package-cson: https://www.npmjs.com/package/cson
 
-== Hacking and Testing the Extension
+This guide describes how to develop changes for this Visual Studio Code plugin and how to test them locally.
 
-. Clone the repository from Github
+== Prerequisites
+
+Before developing changes, install the following prerequisites:
+
+. https://nodejs.org/en/[Node 16 or later]
+. https://code.visualstudio.com/[Visual Studio Code]
+. https://git-scm.com/[Git]
+
+== Developing Changes and Testing the Extension
+
+. Clone the repository from GitHub.
 +
  $ git clone https://github.com/asciidoctor/asciidoctor-vscode.git
 
 . Open the folder within Visual Studio Code.
 
+. Download NPM package dependencies.
++
+ $ npm install
+
 . Make changes (ideally on a separate branch).
 
 Testing is done differently on the desktop client and the web client.
 
-=== Desktop
+=== Running the Desktop version
 
 . With the folder open in Visual Studio code, from the menu bar, select Run > 'Start Debugging' (or click the Run and Debug icon on the left sidebar and click on the green 'Run Extension' play button)
 . A new Visual Studio Code instance will open with '[Extension Development Host]' in the header.
@@ -34,20 +48,20 @@ To run the tests suite, select 'Run Extension Tests' in the 'Run' view then, cli
 You can also run the tests suite from a terminal using:
 
  $ npm t
- 
+
 [NOTE]
 ====
 In that case, you must close your Visual Studio code instance otherwise you will get an error:
 
  Running extension tests from the command line is currently only supported if no other instance of Code is running.
 ====
 
-=== Web
+=== Running the Web version
 
 On Linux to test the extension run the following command within the repository folder from the command line:
 
  $ npm run build-web && npm run build-preview && npx @vscode/test-web --browserType=chromium --extensionDevelopmentPath=${PWD}
- 
+
 IMPORTANT: In development, you will need to disable security because resources are loaded through http://localhost:3000/static/devextensions
 
 TIP: Looking at the browser's Web Developer tools can often help locate errors.