added shape_cyclicpolygon

docs/lib-shape_cyclicpolygon.md

@@ -0,0 +1,58 @@
+# shape_cyclicpolygon
+
+Returns shape points of a regular cyclic polygon. They can be used with xxx_extrude modules of dotSCAD. The shape points can be also used with the built-in polygon module. 
+
+## Parameters
+
+- `sides` : The radius of the circle.
+- `circle_r` : The radius of the circumcircle.
+- `$fa`, `$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_cyclicpolygon.scad>;
+
+    circle_r = 10;
+    corner_r = 3;
+
+    $fn = 24;
+
+    for(i = [0:2]) {
+        translate([i * circle_r * 2, 0, 0]) 
+            polygon(
+                shape_cyclicpolygon(
+                    sides = 3 + i, 
+                    circle_r = circle_r, 
+                    corner_r = corner_r
+                )
+            );
+    }
+
+    for(i = [0:2]) {
+        translate([i * circle_r * 2, -circle_r * 2, 0]) 
+            polygon(
+                shape_cyclicpolygon(
+                    sides = 4 + i, 
+                    circle_r = circle_r , 
+                    corner_r = corner_r
+                )
+            );
+    }
+
+![shape_cyclicpolygon](images/lib-shape_cyclicpolygon-1.JPG)
+
+    include <shape_cyclicpolygon.scad>;
+    include <rotate_p.scad>;
+    include <cross_sections.scad>;
+    include <polysections.scad>;
+    include <ring_extrude.scad>;
+
+    shape_pts = shape_cyclicpolygon(
+        sides = 5, 
+        circle_r = 10, 
+        corner_r = 3
+    );
+
+    ring_extrude(shape_pts, radius = 20, angle = 180, twist = 90);
+
+![shape_cyclicpolygon](images/lib-shape_cyclicpolygon-2.JPG)