add doc

README.md

@@ -133,7 +133,7 @@ These examples incubate dotSCAD and dotSCAD refactors these examples. See [examp
 - [sphere_spiral_extrude](https://openhome.cc/eGossip/OpenSCAD/lib2x-sphere_spiral_extrude.html)
 
 ### Util
-- util/angle_between
+- [util/angle_between](https://openhome.cc/eGossip/OpenSCAD/lib3x-angle_between.html)
 - [util/bsearch](https://openhome.cc/eGossip/OpenSCAD/lib3x-bsearch.html)	
 - [util/choose](https://openhome.cc/eGossip/OpenSCAD/lib3x-choose.html)
 - [util/dedup](https://openhome.cc/eGossip/OpenSCAD/lib3x-dedup.html)

docs/lib3x-angle_between.md

@@ -0,0 +1,18 @@
+# angle_between
+
+Returns the angle between two vectors.
+
+**Since:** 3.0
+
+## Parameters
+
+- `vt1` : vector 1.
+- `vt2` : vector 2.
+
+## Examples
+
+    use <util/angle_between.scad>;
+
+    assert(angle_between([0, 1], [1, 0]) == 90);
+    assert(angle_between([0, 1, 0], [1, 0, 0]) == 90);
+    assert(round(angle_between([1, 1, 0], [1, 1, sqrt(2)])) == 45);

src/util/angle_between.scad

@@ -1 +1,11 @@
+/**
+* angle_between.scad
+*
+* @copyright Justin Lin, 2020
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib3x-angle_between.html
+*
+**/
+
 function angle_between(vt1, vt2) = acos((vt1 * vt2) / (norm(vt1) * norm(vt2)));