Add gulp preview task as npm script (PR #42)

Author: ggrossetie

README.adoc

@@ -156,6 +156,13 @@ To build the UI and preview it in a local web server, run the `preview` command:
 
  $ gulp preview
 
+[TIP]
+====
+Alternatively, you can use the following command:
+
+ $ npm start
+====
+
 You'll see a URL listed in the output of this command:
 
 ....

package.json

@@ -7,6 +7,9 @@
     "type": "git",
     "url": "https://github.com/asciidoctor/asciidoctor-docs-ui"
   },
+  "scripts": {
+    "start": "gulp preview"
+  },
   "engines": {
     "node": ">= 10.17.0"
   },