add cone doc

docs/lib2-cone.md

@@ -0,0 +1,67 @@
+# cone
+
+Create a cone for rotatable models.
+
+![cone](images/lib2-cone-1.JPG)
+
+**Since:** 2.1
+
+
+## Parameters
+
+- `radius` : The radius of the cone.
+- `length` : The length of the cone.
+- `spacing` : The spacing between the cone and void component. Default to `0.5`.
+- `angle` : The angle between the bottom and hypotenuse. Default to `50` degree.
+- `void` : The void component for differencing. It's used to create a cavity for a cone. Default to `false`.
+- `ends` : Default to `false`. To create a double-ended cone, set it to `true`.
+
+## Examples
+
+	include <part/cone.scad>;
+
+	radius = 2.5;
+    length = 2;
+	spacing = 0.5;
+
+	$fn = 48;
+
+	cone(
+		radius = radius, 
+		spacing = spacing,
+        length = 2
+	);
+
+	%cone(
+		radius = radius, 
+		spacing = spacing,
+        length = 2,		
+        void = true
+	);
+
+![cone](images/lib2-cone-2.JPG)
+
+	include <part/cone.scad>;
+
+	radius = 2.5;
+    length = 2;
+	spacing = 0.5;
+
+	$fn = 48;
+
+	cone(
+		radius = radius, 
+		spacing = spacing,
+        length = 2,
+        ends = true
+	);
+
+	%cone(
+		radius = radius, 
+		spacing = spacing,
+        length = 2,		
+        void = true,
+        ends = true
+	);
+
+![cone](images/lib2-cone-3.JPG)