docs: add example demo page

Author: DJTB

package.json

@@ -8,6 +8,7 @@
     "email": "[email protected]"
   },
   "repository": "https://github.com/DJTB/hatsuon",
+  "homepage": "https://djtb.github.io/hatsuon",
   "engines": {
     "node": ">=8"
   },

readme.md

@@ -21,6 +21,10 @@ Japanese dictionaries often display the pitch accent of a word with a single num
 npm install --save hatsuon
 ```
 
+## Demo
+
+[Visualization Example](https://djtb.github.io/hatsuon)
+
 ## Usage
 
 ```js