Add JavaScript HTMLProcessor class (#58)

This class provides extended capabilities when applying to DOM,
but with larger code size. Please see the JSDoc for more details.

* tushuhei review

javascript/README.md

@@ -63,6 +63,25 @@ console.log(ele.outerHTML);
 // <p class="budou-this" style="word-break: keep-all; overflow-wrap: break-word;">今日は<b><wbr>とても<wbr>天気</b>です。</p>
 ```
 
+There is another way to apply the process to an HTML element.
+
+```javascript
+import { HTMLProcessor } from 'budoux';
+const ele = document.querySelector('p.budou-this');
+const applier = new HTMLProcessor(parser);
+applier.applyToElement(ele);
+```
+
+The [`HTMLProcessor`] class
+recognizes separate or nested paragraphs more correctly,
+its output is generally more memory efficient for browsers,
+and it can customize its output such as inserting a space at boundaries
+which is often useful for accessibility,
+but the bundle code size is larger.
+Please see the JSDoc for more details.
+
+[`HTMLProcessor`]: https://github.com/google/budoux/blob/main/javascript/src/html_processor.ts
+
 ### Loading a custom model
 
 You can load your own custom model as follows.