update install instructions to cover installation of pro Font Awesome icons (and what happens if you don't)

Author: mojavelinux

README.adoc

@@ -143,6 +143,22 @@ In your terminal, execute the following command:
 This command installs the dependencies listed in [.path]_package.json_ into the [.path]_node_modules/_ folder inside the project.
 This folder does not get included in the UI bundle and should _not_ be committed to the source control repository.
 
+In order for the pro Font Awesome icons work, you must first add the following file to the root folder of the UI project:
+
+..npmrc
+----
+@fortawesome:registry=https://npm.fontawesome.com/
+//npm.fontawesome.com/:_authToken=${FONTAWESOME_NPM_AUTH_TOKEN}
+----
+
+Then pass the token to npm when running the `npm install` command:
+
+FONTAWESOME_NPM_AUTH_TOKEN=xxxx npm install
+
+If you don't supply this token when installing packages, the pro icons will not be installed, but the UI preview will still work.
+Any icon available in the free collection will still be found, but the pro icons will be missing.
+When an icon is missing, a missing icon is shown instead.
+
 === Preview the UI
 
 The UI project is configured to preview offline.