Add a colcon.pkg file for proper ament integration (UniversalRobots#131)

This will help making the package available in the ament build system.
This will effecticely advertise the hooks that we install in our CMakeLists.txt

Author: fmauch

colcon.pkg

@@ -0,0 +1,6 @@
+{
+  "hooks": [
+    "share/ur_client_library/hook/ament_prefix_path.dsv",
+    "share/ur_client_library/hook/ros_package_path.dsv",
+  ]
+}