added bezier

docs/lib-bezier.md

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+# bezier
+
+Given a set of control points, the bezier function returns points of the Bézier path. Combined with the `polyline`, `polyline3d` or `hull_polyline3d` module defined in my lib-openscad, you can create a Bézier curve.
+
+## Parameters
+
+- `t_step` : The distance between two points of the Bézier path.
+- `points` : A set of `[x, y, z]` control points.
+
+## Examples
+
+If you have four control points and combine with the `hull_polyline3d` module:
+
+	t_step = 0.05;
+	width = 2;
+	
+	p0 = [0, 0, 0];
+	p1 = [40, 60, 35];
+	p2 = [-50, 90, 0];
+	p3 = [0, 200, -35];
+	
+	points = bezier(t_step, 
+	    [p0, p1, p2, p3]
+	);
+	
+	hull_polyline3d(points, width);      
+
+![bezier](images/lib-bezier-1.JPG)