update documentation

plugins/linkattacher/README.md

@@ -1,5 +1,5 @@
 ### LinkAttacher Plugin
-The LinkAttacher is a Gazebo _model plugin_. It can be used to attach/detach two _link_ of _model_ spawned in Gazebo environment. The attachment is made by adding a _joint_ between the links specified, with a given _jointType_ (default type is `fixed`). The link of the model we want to attach is the parent link and the link of the robot model (to which this plugin is added) becomes the child link for the newly created joint. So, the new joint is named by the `model_link_name` suffixed with the phrase *_jointType_joint* in the name e.g, l_hand_fixed_joint.
+The LinkAttacher is a Gazebo _model plugin_. It can be used to attach/detach two _link_ of _model_ spawned in Gazebo environment. The attachment is made by adding a _joint_ between the links specified, with a given _jointType_ (available joint types are those in [SDF documentation](http://sdformat.org/spec?ver=1.6&elem=joint#joint_type) with the exception of `gearbox` that is not available). The link of the model we want to attach is the parent link and the link of the robot model (to which this plugin is added) becomes the child link for the newly created joint. So, the new joint is named by the `model_link_name` suffixed with the phrase *_jointType_joint* in the name e.g, l_hand_fixed_joint.
 
 Additionally, this plugin also provides control over _gravity_ of the models spawned in Gazebo environment.
 Currently, this plugin is used for coupling the hands of two robots, or those robot model and human model in pHRI experiments. Also, this plugin can be used to attach objects to the robot links.