Add settings section to readme

Author: iansan5653

README.md

@@ -68,6 +68,15 @@ list.addEventListener('combobox-commit', function(event) {
 
 When a label is clicked on, `click` event is fired from both `<label>` and its associated input `label.control`. Since combobox does not know about the control, `combobox-commit` cannot be used as an indicator of the item's selection state.
 
+## Settings
+
+For advanced configuration, the constructor takes an optional third argument. This is a settings object with the following setting:
+
+<dl>
+  <dt><code>tabInsertsSuggestions: boolean = true</code></dt>
+  <dd>Control whether the highlighted suggestion is inserted when <kbd>Tab</kbd> is pressed.</dd>
+</dl>
+
 ## Development
 
 ```