add polar_coordinate

docs/lib3x-polar_coordinate.md

@@ -0,0 +1,18 @@
+# polar_coordinate
+
+Converts from Cartesian to Polar coordinates.
+
+**Since:** 3.0
+
+## Parameters
+
+- `point` : The Cartesian coordinates (also called rectangular coordinates) of a point.
+
+## Examples
+
+    use <util/polar_coordinate.scad>;
+    
+	coord = polar_coordinate([100, 100]);
+	r = round(coord[0]);
+	theta = round(coord[1]);
+    assert([r, theta] == [141, 45]);