added shape_pie

docs/lib-shape_pie.md

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+# shape_pie
+
+Returns shape points and triangle indexes of a pie (circular sector) shape. They can be used with xxx_extrude modules of dotSCAD. The shape points can be also used with the built-in polygon module. 
+
+## Parameters
+
+- `radius` : The radius of the circle.
+- `angles` : A 2 element vector which defines the central angle. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.
+- `$fa`, `$fs`, `$fn` : Check [the circle module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#circle) for more details.
+
+## Examples
+
+	include <shape_pie.scad>;
+	
+	shape_pts_tris = shape_pie(10, [45, 315], $fn = 24);
+
+![shape_pie](images/lib-shape_pie-1.JPG)
+
+include <shape_pie.scad>;
+include <helix.scad>;
+include <rotate_p.scad>;
+include <cross_sections.scad>;
+include <polysections.scad>;
+include <helix_extrude.scad>;
+
+shape_pts_tris = shape_pie(10, [45, 315], $fn = 8);
+
+helix_extrude(shape_pts_tris[0], 
+    radius = 40, 
+    levels = 5, 
+    level_dist = 20,
+    triangles = shape_pts_tris[1]
+);
+
+![shape_pie](images/lib-shape_pie-2.JPG)