add doc

docs/lib2-joint_T.md

@@ -0,0 +1,32 @@
+# joint_T
+
+Create a joint_T for rotatable models.
+
+![joint_T](images/lib2-joint_T-1.JPG)
+
+**Since:** 2.1
+
+## Parameters
+
+- `shaft_r` : The shaft radius of the joint_T.
+- `shaft_h` : The height of the shaft.
+- `t_leng` : The length of the T part.
+- `thickness` : The thicnkess of the ring.
+- `spacing` : The spacing between the shaft and the ring. Default to `0.5`.
+- `center` : `false` (default) for z ranges from 0 to `shaft_h`. `true` for z ranges from `-shaft_h / 2` to `shaft_h / 2`.
+
+## Examples
+
+	include <part/joint_T.scad>;
+
+	$fn = 48;
+
+	joint_T(
+		shaft_r = 2,
+		shaft_h = 15,
+		t_leng = 15,
+		thickness = 2,
+		spacing = 0.5
+	);
+
+![joint_T](images/lib2-joint_T-2.JPG)