add m_shearing doc

docs/lib-m_shearing.md

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+# m_shearing
+
+Generate a 4x4 transformation matrix which can pass into `multmatrix` to shear all child elements along the X-axis, Y-axis, or Z-axis in 3D.
+
+## Parameters
+
+- `sx` : An array `[SHy, SHz]`. The new coordinates of child elements are `(x + SHy * y + SHz * z, y, z)`.
+- `sy` : An array `[SHx, SHz]`. The new coordinates of child elements are `(x, y + SHx * x + SHz * z, z)`.
+- `sz` : An array `[SHx, SHy]`. The new coordinates of child elements are `(x, y, z + SHx * x + SHy * y)`.
+
+## Examples
+
+	include <m_shearing.scad>;
+
+	color("red") {
+		multmatrix(m_shearing(sx = [1, 0]))
+			cube(1);
+			
+		translate([2, 0, 0]) multmatrix(m_shearing(sx = [0, 1]))
+			cube(1);
+			
+		translate([4, 0, 0]) multmatrix(m_shearing(sx = [1, 1]))
+			cube(1);
+	}
+
+	translate([0, -3, 0]) color("green") {
+		multmatrix(m_shearing(sy = [1, 0]))
+			cube(1);
+			
+		translate([2, 0, 0]) multmatrix(m_shearing(sy = [0, 1]))
+			cube(1);
+			
+		translate([4, 0, 0]) multmatrix(m_shearing(sy = [1, 1]))
+			cube(1);
+	}
+
+	translate([0, -5, 0]) color("blue") {
+		multmatrix(m_shearing(sz = [1, 0]))
+			cube(1);
+			
+		translate([2, 0, 0]) multmatrix(m_shearing(sz = [0, 1]))
+			cube(1);
+			
+		translate([4, 0, 0]) multmatrix(m_shearing(sz = [1, 1]))
+			cube(1);
+	}
+
+![m_shearing](images/lib-m_shearing-1.JPG)
+