Add initial touch bar docs

docs/api/touch-bar-slider.md

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+## Class: TouchBarSlider
+
+> Create a slider in the touch bar for native macOS applications
+
+Process: [Main](../tutorial/quick-start.md#main-process)
+
+### `new TouchBarSlider(options)`
+
+* `options` Object
+  * `label` String (optional) - Label text.
+  * `value` Integer (optional) - Selected value.
+  * `minValue` Integer (optional) - Minimum value.
+  * `maxValue` Integer (optional) - Maximum value.
+  * `change` Function (optional) - Function to call when the slider is changed.
+
+### Instance Properties
+
+The following properties are available on instances of `TouchBarSlider`:
+
+#### `touchBarSlider.label`
+
+The slider's current text. Changing this value immediately updates the slider
+in the touch bar.
+
+#### `touchBarSlider.value`
+
+The slider's current value. Changing this value immediately updates the slider
+in the touch bar.
+
+#### `touchBarSlider.minValue`
+
+The slider's current minimum value. Changing this value immediately updates the
+slider in the touch bar.
+
+#### `touchBarSlider.maxValue`
+
+The slider's current maximum value. Changing this value immediately updates the
+slider in the touch bar.