add doc

README.md

@@ -370,7 +370,7 @@ These examples incubate dotSCAD and dotSCAD refactors these examples. See [examp
  Signature | Description
 --|--
 [**select**(i)](https://openhome.cc/eGossip/OpenSCAD/lib3x-select.html) | select module objects.
-polyline_join | place a join on each point. Hull each pair of joins and union all convex hulls.
+[`polyline_join(points)`](https://openhome.cc/eGossip/OpenSCAD/lib3x-polyline_join.html) | place a join on each point. Hull each pair of joins and union all convex hulls.
 
 ## Polyhedra
 

docs/lib3x-polyline_join.md

@@ -0,0 +1,40 @@
+# polyline_join
+
+Place a join on each point. Hull each pair of joins and union all convex hulls.
+
+## Parameters
+
+- `points` : a list of points.
+
+## Examples
+  
+    use <polyline_join.scad>;
+    
+    polyline_join([[0, 0], [10, 0], [10, 10], [0, 10]]) 
+        square(1);
+        
+    polyline_join([[15, 0], [25, 0], [25, 10], [15, 10]]) {
+        square(1);
+        circle(1);
+        square(1);
+        circle(2);
+    }
+
+![polyline_join](images/lib3x-polyline_join-1.JPG)
+
+    use <polyline_join.scad>;
+        
+    polyline_join([[0, 0, 0], [10, 0, 0], [10, 0, 10], [10, 10, 10]]) 
+        cube(1);
+        
+    polyline_join([[15, 0, 0], [25, 0, 0], [25, 0, 10], [25, 10, 10]]) {
+        cube(1);
+        sphere(1);
+        cube(1);
+        sphere(2);
+    }
+
+![polyline_join](images/lib3x-polyline_join-2.JPG)
+
+
+