add bend_extrude doc

docs/lib-bend_extrude.md

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+# bend_extrude
+
+The purpose of `bend_extrude` is to replace `bend` when you have a 2D shape. `bend_extrude` is faster and doesn't produce jagged edges.
+
+**Since 1.3**
+
+## Parameters
+
+- `size` : The size of a square which can contain the target shape.
+- `thickness` : The thinkness used to extrude the shape.
+- `angle` : The central angle of the arc shape. The radius of the arc is calculated automatically.
+- `frags` : Number of fragments. The target shape will be cut into `frags` fragments and recombined into an arc object. The default value is 24.
+
+## Examples
+
+The containing square of the target shape should be laid down on the x-y plane. For example.
+
+	include <bend_extrude.scad>;
+
+	x = 9.25;
+	y = 9.55;
+
+	%square(size = [x, y]);
+	text("A");
+
+![bend_extrude](images/lib-bend_extrude-1.JPG)
+
+Once you have the size of the containing square, you can use it as the `size` argument of the `bend_extrude` module.
+
+    include <bend.scad>;
+
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	       
+	*cube(size = [x, y, z]);
+	
+	bend(size = [x, y, z], angle = 270)
+	    linear_extrude(z) text("A");
+
+![bend_extrude](images/lib-bend_extrude-2.JPG)
+
+The arc shape is smoother if the `frags` value is larger. 
+
+    include <bend.scad>;
+	
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	
+	bend(size = [x, y, z], angle = 270, frags = 360)
+	    linear_extrude(z) 
+	        text("A");
+
+![bend](images/lib-bend-3.JPG)
+
+This module is especially useful when you want to create things such as [Voronoi bracelet](https://www.thingiverse.com/thing:3650115).