add doc

docs/lib3x-m_transpose.md

@@ -0,0 +1,30 @@
+# m_transpose
+
+It transposes a matrix.
+
+**Since:** 2.4
+
+## Parameters
+
+- `m` : A matrix.
+
+## Examples
+
+    use <matrix/m_transpose.scad>;
+
+    original_m = [
+        [1, 2, 3, 4],
+        [5, 6, 7, 8],
+        [9, 10, 11, 12]
+    ];
+
+    traget = [
+        [1, 5, 9],
+        [2, 6, 10],
+        [3, 7, 11],
+        [4, 8, 12]
+    ];
+
+    transposed = m_transpose(original_m);
+
+    assert(transposed == traget);