add doc

docs/lib3x-polyline_join.md

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+# 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)
+
+
+