add has doc

docs/lib2x-has.md

@@ -0,0 +1,25 @@
+# has
+
+If `lt` contains `elem`, this function returns `true`. If you want to test elements repeatly, sorting `lt` first and setting `sorted` to `true` will be faster.
+
+**Since:** 2.3
+
+## Parameters
+
+- `lt` : A list of vectors.
+- `elem` : A vector.
+- `sorted` : If `false`, use native `search`. If `true`, `lt` must be sorted by zyx (from the last idx to first) and `has` will use binary search internally.
+
+## Examples
+
+    use <pixel/px_circle.scad>;
+    use <util/sort.scad>;
+    use <util/has.scad>;
+
+    pts = px_circle(10);
+    assert(has(pts, [2, -10])); 
+    assert(!has(pts, [0, 0]));  
+
+    sorted_pts = sort(sort(pts, by = "x"), by = "y");
+    assert(has(sorted_pts, [2, -10]));
+    assert(!has(sorted_pts, [0, 0]));