Add initial touch bar docs

docs/api/touch-bar-label.md

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+## Class: TouchBarLabel
+
+> Create a label in the touch bar for native macOS applications
+
+Process: [Main](../tutorial/quick-start.md#main-process)
+
+### `new TouchBarLabel(options)`
+
+* `options` Object
+  * `label` String (optional) - Text to display.
+
+### Instance Properties
+
+The following properties are available on instances of `TouchBarLabel`:
+
+#### `touchBarLabel.label`
+
+The label's current text. Changing this value immediately updates the label in
+the touch bar.