How to create a new plane model for Gazebo (SITL/Mavproxy/Gazebo)

Hi @Nicolas_T, we recently had a request for a v-tail in Gazebo (V-tail gazebo SITL QGroundControl).

The PR containing the model is here: Gazebo: add X-UAV Mini Talon V-Tail by srmainwaring · Pull Request #98 · ArduPilot/SITL_Models · GitHub.