added bend

docs/lib-bend.md

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+# bend
+
+Bend a 3D object into an arc shape.
+
+## Parameters
+
+- `size` : The size of a cube which can contain the target object.
+- `angle` : The central angle of the arc shape. The radius of the arc is calculated automatically.
+- `fn` : Number of fragments. The target object will be cut into `fn` fragments and recombined into an arc shape.
+
+## Examples
+
+The containing cube of the target object should be laid down on the x-y plane. For examples.
+
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	       
+	%cube(size = [x, y, z]);
+	linear_extrude(z) text("A");
+
+![bend](images/lib-bend-1.JPG)
+
+Once you have the size of the containing cube, you can use it as the `size` argument of the `bend` module.
+
+	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](images/lib-bend-2.JPG)
+
+The arc shape is smoother if the `frags` is larger. 
+	
+	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 [zentangle bracelet](https://www.thingiverse.com/thing:1569263).
+
+[![zentangle bracelet](http://thingiverse-production-new.s3.amazonaws.com/renders/eb/93/4f/62/1f/3bd1f628e1e566dcb5313035e4f3345b_preview_featured.JPG)](https://www.thingiverse.com/thing:1569263)