Add LGPLv3 license to package.xml. (#21)

Add comment LGPLv3 is only for `rqt_dotgraph/xdot_qt.py`. All other
files are covered by CC0 license.

package.xml

@@ -7,7 +7,8 @@
   <maintainer email="[email protected]">Alexander Xydes</maintainer>
   <maintainer email="[email protected]">Thomas Denewiler</maintainer>
   <maintainer email="[email protected]">Greg Kogut</maintainer>
-  <license>CC0</license>
+  <license>LGPLv3</license> <!-- For rqt_dotgraph/xdot_qt.py only. -->
+  <license>CC0</license> <!-- For all other files. -->
 
   <exec_depend>ament_index_python</exec_depend>
   <exec_depend>python3-pyside2</exec_depend>