add doc

docs/lib-m_cumulate.md

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+# m_cumulate
+
+The power of using transformation matrice is that you can cumulate all transformations in a matrix. This function multipies all transformation matrice. 
+
+**Since:** 1.1
+
+## Parameters
+
+- `matrice` : A list of 4x4 transformation matrice.
+
+## Examples
+
+	include <m_rotation.scad>;
+	include <m_scaling.scad>;
+	include <m_translation.scad>;
+	include <m_cumulate.scad>
+
+	m = m_cumulate([
+		m_translation([10, 20, 10]), m_scaling(2), m_rotation(60)]
+	);
+
+	multmatrix(m) 
+		cube(1);
+		
+	multmatrix(m)    
+		sphere(1);
+
+
+![m_cumulate](images/lib-m_cumulate-1.JPG)
+

src/m_cumulate.scad

@@ -1,3 +1,13 @@
+/**
+* m_cumulate.scad
+*
+* @copyright Justin Lin, 2019
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-m_cumulate.html
+*
+**/
+
 include <__private__/__m_multiply.scad>;
 
 function _m_cumulate(matrice, i) =