Add a package.xml to enable colcon to build as a ros2 package #42

Hi, LG Electronics,

I'm contributing some work to autoware.auto based on this PythonAPI. Suffice to say, we hope to use colcon to build this project as a ROS2 package. Although it works currently, it lacks of a package.xml file to be a standard ROS2 package. More details could be found at here. For some reasons, I couldn't directly submit a pull request. So I leave the content of package.xml file in this issue and ask Josh Whitely to help me with it. Thanks!

The content in packge.xml is shown as below. It depends on some packages which could be installed by rosdep. However, pull request hasn't been accepted by rosdistro. It should work when it's accepted. Another thing is I leave maintainer section to be TO DO. Could you give information on this section please? Thanks a lot!

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>lgsvl_python3_api</name>
  <version>1.0.0</version>
  <description>A Python API for SVL Simulator</description>
  <maintainer email="TO DO">TO DO</maintainer>
  <license>non-standard</license>

  <depend>python3-environs-pip</depend>
  <depend>python3-websocket</depend>
  <depend>python3-websockets</depend>

  <test_depend>ament_copyright</test_depend>
  <test_depend>ament_flake8</test_depend>
  <test_depend>ament_pep257</test_depend>
  <test_depend>python3-pytest</test_depend>

  <export>
    <build_type>ament_python</build_type>
  </export>
</package>