Add initial touch bar docs

docs/api/touch-bar-button.md

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+## Class: TouchBarButton
+
+> Create a button in the touch bar for native macOS applications
+
+Process: [Main](../tutorial/quick-start.md#main-process)
+
+### `new TouchBarButton(options)`
+
+* `options` Object
+  * `label` String (optional) - Button text.
+  * `backgroundColor` String (optional) - Button background color in hex format,
+    i.e `#ABCDEF`.
+  * `icon` NativeImage (optional) - Button icon.
+  * `click` Function (optional) - Function to call when the button is clicked.
+
+### Instance Properties
+
+The following properties are available on instances of `TouchBarButton`:
+
+#### `touchBarButton.label`
+
+The button's current text. Changing this value immediately updates the button
+in the touch bar.
+
+#### `touchBarButton.backgroundColor`
+
+The button's current background color. Changing this value immediately updates
+the button in the touch bar.
+
+#### `touchBarButton.icon`
+
+The button's current icon. Changing this value immediately updates the button
+in the touch bar.