maint: added minmaxslider package

Assets/SimpleMinMaxSlider/Scripts/MinMaxSliderAttribute.cs

@@ -0,0 +1,16 @@
+namespace GD.MinMaxSlider
+{
+    using UnityEngine;
+
+    public class MinMaxSliderAttribute : PropertyAttribute{
+
+        public float min;
+        public float max;
+
+        public MinMaxSliderAttribute(float min, float max)
+        {
+            this.min = min;
+            this.max = max;
+        }
+    }  
+}