added archimedean_spiral

docs/lib-archimedean_spiral.md

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+# archimedean_spiral
+
+Get all points and angles on the path of an archimedean_spiral. The distance between two  points is almost constant. 
+
+It returns a vector of `[[x, y], angle]`. 
+
+An `init_angle` less than 180 degrees is not recommended because the function uses an approximate approach. If you really want an `init_angle` less than 180 degrees, a larger `arm_distance` is required. To reduce the error value at the calculated distance between two points, you may try a smaller `point_distance`.
+
+## Parameters
+
+- `arm_distance` : If any ray from the origin intersects two successive turnings of the spiral, we'll have two points. The `arm_distance` is the distance between these two points.
+- `init_angle` : In polar coordinates `(r, θ)` Archimedean spiral can be described by the equation `r = bθ ` where `θ` is measured in radians. For being consistent with OpenSCAD, the function here use degrees. The `init_angle` is which angle the first point want to start.
+- `point_distance` : Distance between two points on the path.
+- `num_of_points` : How many points do you want?
+
+## Examples
+    
+	include <polyline2d.scad>;
+	
+	points_angles = archimedean_spiral(
+	    arm_distance = 10,
+	    init_angle = 180,
+	    point_distance = 5,
+	    num_of_points = 100 
+	); 
+	
+	points = [for(pa = points_angles) pa[0]];
+	
+	polyline2d(points, width = 1);
+
+
+![archimedean_spiral](images/lib-archimedean_spiral-1.JPG)
+	
+	points_angles = archimedean_spiral(
+	    arm_distance = 10,  
+	    init_angle = 180, 
+	    point_distance = 5,
+	    num_of_points = 100 
+	); 
+	
+	for(pa = points_angles) {
+	    translate(pa[0]) 
+	        circle(2);
+	}
+
+![archimedean_spiral](images/lib-archimedean_spiral-2.JPG)
+
+t = "3.141592653589793238462643383279502884197169399375105820974944592307816406286";
+
+	points = archimedean_spiral(
+	    arm_distance = 15,
+	    init_angle = 450, 
+	    point_distance = 12, 
+	    num_of_points = len(t) 
+	); 
+	
+	for(i = [0: len(points) - 1]) {
+	    translate(points[i][0])          
+	        rotate(points[i][1] + 90)  
+	            text(t[i], valign = "center", halign = "center");
+	}
+
+![archimedean_spiral](images/lib-archimedean_spiral-3.JPG)
+
+