Added ResetSelectableHighlight documentation

Author: sjackson-ethar

Controls/Images/ResetSelectableHighlight-demo.gif

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+# ResetSelectableHighlight
+
+Handy Selectable script to un-highlight a selectable component in Unity (e.g. a Button) when the user moves away from it, EVEN IF the user has holding a button on it.
+
+Resolves the situation where Unity UI Components remain in a highlighted state even after the pointer has moved away (e.g. user holding a button, mouse, pointer down).
+Now whenever the cursor leaves the component, it will force the UI component to revert to un-highlighted.
+
+---------
+
+## Contents
+
+> 1 [Overview](#overview)
+>
+> 2 [Usage](#usage)
+>
+> 3 [Credits and Donation](#credits-and-donation)
+
+---------
+
+## Overview
+
+An addon component that forces a Selectable Control (Button, InputField, etc) to reset its highlighted state whenever the mouse cursor leaves the control.
+
+> By default in the Unity UI, if a pointer is held down when the cursor leaves the control, it remains in a highlighted state.
+
+![Demo comparing the ResetSelectable against the default Unity UI Behaviour](Images/ResetSelectableHighlight-demo.gif)
+
+Button on the left has the ResetSelectableHighlight component, while the button on the right is a default Unity UI Button, Mouse is held down when moving off the control.
+
+---------
+
+## Usage
+
+Use as follows:
+
+* "Add Component -> Extensions -> *ResetSelectableHighlight*"
+
+This will add the ResetSelectableHighlight component to the selected GameObject.
+
+> No config required.
+
+---------
+
+## Credits and Donation
+
+SimonDarksideJ