updated docs

docs/lib-helix_extrude.md

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+# helix_extrude
+
+Extrudes a 2D shape along a helix path. 
+
+When using this module, you should use points to represent the 2D shape. You need to provide indexes of triangles, too. This module provides two prepared triangles indexes. One is `"RADIAL"`. See [polysections](https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html) for details.
+
+Its `$fa`, `$fs` and `$fn` parameters are consistent with the `cylinder` module. 
+
+Dependencies: `circle_path`, `helix`, `rotate_p.scad`, `cross_sections`, `polysections`.
+
+## Parameters
+
+- `shape_pts` : A list of points represent a shape. See the example below.
+- `radius` : The radius of the cylinder.
+- `levels` : The level count is performed every 360 degrees. 
+- `level_dist` : The distance between two vertial points.
+- `vt_dir` : `"SPI_DOWN"` for spiraling down. `"SPI_UP"` for spiraling up. The default value is `"SPI_DOWN"`.
+- `rt_dir` : `"CT_CLK"` for counterclockwise. `"CLK"` for clockwise. The default value is `"CT_CLK"`.
+- `twist` : The number of degrees of through which the shape is extruded.
+- `scale` : Scales the 2D shape by this value over the length of the extrusion. Scale can be a scalar or a vector.
+- `triangles` : `"RADIAL"` (default), `"HOLLOW"` or user-defined indexes. See [polysections](https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html) for details.
+- `$fa`, `$fs`, `$fn` : Check [the cylinder module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#cylinder) for more details.
+
+## Examples
+    
+	include <circle_path.scad>;
+	include <helix.scad>;
+	include <rotate_p.scad>;
+	include <cross_sections.scad>;
+	include <polysections.scad>;
+	include <helix_extrude.scad>;
+
+	shape_pts = [
+		[-5, -2], [-5, 2],
+		[-4, 2], [-4, 0],
+		[4, 0], [4, 2],
+		[5, 2], [5, -2]
+	];
+
+	helix_extrude(shape_pts, 
+		radius = 40, 
+		levels = 5, 
+		level_dist = 10,
+		triangles = [
+			[0, 1, 2],
+			[0, 2, 3],
+			[0, 3, 4],
+			[0, 4, 7],
+			[4, 5, 6],
+			[4, 6, 7]
+		]
+	);
+
+![helix_extrude](images/lib-helix_extrude-1.JPG)
+