Add sliders documentation

Author: Julia Schwarz

Documentation/Images/Sliders/MRTK_Sliders_Main.png

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+# Sliders
+![Sliders](../Documentation/Images/Sliders/MRTK_Sliders_Main.png)
+
+Sliders are UI components that allow you to continuously change a value by moving a slider on a track. Currently the Pinch Slider can be moved by directly grabbing the slider, either directly or at a distance. Sliders work on AR and VR, using motion controllers, hands, or Gesture + Voice.
+
+## Sliders Example
+You can see an example usage of sliders in the file `Assets\MixedRealityToolkit.Examples\Demos\UX\Slider\Scenes\SliderExample.unity`.
+
+## How to use Sliders
+Drag and drop the **PinchSlider** prefab into the scene hierarchy. If you want to modify or create your own slider, remember to do the following:
+
+- Make sure your that your thumb object has a collider on it. In the PinchSlider prefab, the collider is on `SliderThumb/Button_AnimationContainer/Slider_Button`
+- Make sure that the object containing the collider also has a Near Interaction Grabbable component on it, if you want to be able to grab the slider near.
+
+We also recommend using the following hierarchy
+
+- PinchSlider - Contains the sliderComponent
+  - SliderThumb - Contains the movable thumb
+  - TrackVisuals - Containing the track and any other visuals
+  - OtherVisuals - Containing any other visuals
+
+## Slider Events
+Sliders expose the following events:
+- OnValueUpdated - Called whenever the slider value changes
+- OnInteractionStarted - Called when the user grabs the slider
+- OnInteractionEnded - Called when the user releases the slider
+- OnHoverEntered - Called when the user's hand / controller hovers over the slider, using either near or far interaction.
+- OnHoverExited - Called when the user's hand / controller is no longer near the slider.
+
+## Configuring Slider Bound and Axis
+You can directly move the starting and end points of the slider by moving the handles in the Scene:
+
+![Sliders Config](../Documentation/Images/Sliders/MRTK_Sliders_Setup.png)
+
+You can also specify the axis (in local space) of the slider via the _Slider Axis_ field
+
+If you cannot use the handles, you can instead specify the start and end points of the slider via the _Slider Start Position_ and _Slider End Position_ fields.
+
+> Note that the slider start and end position fields will get automatically projected to the slider axis.
+
+