Add license comment to package.xml (#107)

* Add license comment to package.xml

* Update license comment

as suggested by law department.

Co-authored-by: Rune Søe-Knudsen <41109954+urrsk@users.noreply.github.com>

package.xml

@@ -25,6 +25,7 @@
   <maintainer email="denis@stoglrobotics.de">Denis Stogl</maintainer>
 
   <license file="LICENSE">BSD-3-Clause</license>
+  <!-- The UR20 meshes constitutes “Graphical Documentation” the use of which is subject to and governed by our “Terms and Conditions for use of Graphical Documentation”, which can be found here: https://www.universal-robots.com/legal/terms-and-conditions/terms_and_conditions_for_use_of_graphical_documentation.txt -->
   <license file="meshes/ur20/LICENSE.txt">Universal Robots A/S’ Terms and Conditions for Use of Graphical Documentation</license>
   <!--<license file="meshes/ur20/LICENSE.txt" source-files="meshes/ur20/*" type="freeform">Universal Robots A/S’ Terms and Conditions for Use of Graphical Documentation</license>-->