Updating instructions for URDF repository (#71)

* 1. Adding images for URDF tutorial 2. Adding instructions highlighting Disable Collison tag

* Adding link to example of disable collision tag in the niryo_one URDF file

tutorials/urdf_importer/urdf_appendix.md

 
 ![](images/ConvexMeshComparison.png)
 
+## Disable Collision Support
+URDF defines the robot visually using Visual Meshes and its inertial volume using collision meshes. Inertial meshes used to define the physical volume of the links and help in calculating the inertia of the links and  detecting the collisions between different physical objects. When a collider mesh is imported in Unity, it is decomposed into near convex shapes to form a concave hull. This is necessary in detecting collisions between two mesh colliders. The changed shape might intersect with each other creating a hindrance in robot movement. To remedy this, we support a ```disable collision``` tag in URDF. To add an exception for collision detection in Unity: 
+1. Identify the links between which you want to ignore the collisions.
+2. Add a tag in the URDF file with the format 
+```XML
+ <disable_collision link1= <name_of_link_1> link2=<name_of_link_2>>
+  </disable_collision>
+```
+An example of the tag can be seen [here](https://github.com/Unity-Technologies/Unity-Robotics-Hub/blob/main/tutorials/pick_and_place/PickAndPlaceProject/Assets/URDF/niryo_one/niryo_one.urdf#L223).
+The disable collision tag flags the links that need to be ignored to the URDF parser. Values of link1 and link2 attributes are the names of the two links between which the collision needs to be ignored. Make sure the names of the links match the names defined in the URDF file.
+
+Note: You can also manually ignore collisions in Unity using [APIs](https://docs.unity3d.com/ScriptReference/Physics.IgnoreCollision.html).