add doc

README.md

@@ -227,7 +227,7 @@ These examples incubate dotSCAD and dotSCAD refactors these examples. See [examp
 
 ### 2D Module
 
-- polygon_hull
+- [polygon_hull](https://openhome.cc/eGossip/OpenSCAD/lib2x-polygon_hull.html)
 
 ### 3D Module
 

docs/lib2x-polygon_hull.md

@@ -0,0 +1,23 @@
+# polyhedron_hull
+
+Create a convex polygon by hulling a list of points. It avoids using `hull` and small 2D primitives to create the polygon.
+
+**Since:** 2.5
+
+## Parameters
+
+- `points` : A list of 2D points.
+
+## Examples
+
+	use <polygon_hull.scad>;
+
+	polygon_hull([
+		[1, 1],
+		[1, 0],
+		[0, 1],
+		[-2, 1],
+		[-1, -1]
+	]);
+
+![polygon_hull](images/lib2x-polygon_hull-1.JPG)

src/polygon_hull.scad

@@ -1,3 +1,13 @@
+/**
+* polygon_hull.scad
+*
+* @copyright Justin Lin, 2020
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib2x-polygon_hull.html
+*
+**/ 
+
 use <__comm__/_convex_hull2.scad>;
 
 module polygon_hull(points) {