add m_rotation doc

docs/lib-m_rotation.md

@@ -0,0 +1,43 @@
+# m_rotation
+
+Generate a 4x4 transformation matrix which can pass into `multmatrix` to rotate the child element about the axis of the coordinate system or around an arbitrary axis. 
+
+## Parameters
+
+- `a` : If it's `[deg_x, deg_y, deg_z]`, the rotation is applied in the order `x`, `y`, `z`. If it's `[deg_x, deg_y]`, the rotation is applied in the order `x`, `y`.  If it's`[deg_x]`, the rotation is only applied to the `x` axis. If it's an number, the rotation is only applied to the `z` axis or an arbitrary axis.
+- `v`: A vector allows you to set an arbitrary axis about which the object will be rotated. When `a` is an array, the `v` argument is ignored. 
+
+## Examples
+
+	include <m_rotation.scad>;
+
+	point = [20, 0, 0];
+	a = [0, -45, 45];
+
+	hull() {
+		sphere(1);
+		multmatrix(m_rotation(a))    
+			translate(point) 
+				sphere(1);   
+	}  
+
+![m_rotation](images/lib-m_rotation-1.JPG)
+
+	include <m_rotation.scad>;
+
+	v = [10, 10, 10];
+
+	hull() {
+		sphere(1);
+		translate(v)
+		sphere(1);   
+	}
+
+	p = [10, 10, 0];
+	for(i = [0:20:340]) {
+		multmatrix(m_rotation(a = i, v = v))
+			translate(p) 
+				sphere(1);  
+	}
+
+![m_rotation](images/lib-m_rotation-2.JPG)