add lib-shear doc

docs/lib-shear.md

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+# shear
+
+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 <shear.scad>;
+
+	color("red") {
+		shear(sx = [1, 0])
+			cube(1);
+			
+		translate([2, 0, 0]) shear(sx = [0, 1])
+			cube(1);
+			
+		translate([4, 0, 0]) shear(sx = [1, 1])
+			cube(1);
+	}
+
+	translate([0, -3, 0]) color("green") {
+		shear(sy = [1, 0])
+			cube(1);
+			
+		translate([2, 0, 0]) shear(sy = [0, 1])
+			cube(1);
+			
+		translate([4, 0, 0]) shear(sy = [1, 1])
+			cube(1);
+	}
+
+	translate([0, -5, 0]) color("blue") {
+		shear(sz = [1, 0])
+			cube(1);
+			
+		translate([2, 0, 0]) shear(sz = [0, 1])
+			cube(1);
+			
+		translate([4, 0, 0]) shear(sz = [1, 1])
+			cube(1);
+	}
+
+![shear](images/lib-shear-1.JPG)
+