Merge pull request #40 from lucasfourier/Updating_TODO

Update TODO.md to add modified equations of motion supporting hybrid and liquid rockets.

Author: giovaniceotto

TODO.md

@@ -4,6 +4,7 @@
 | ----------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------- | -------------------------------- |
 | Rail Buttons                              | Finish rail buttons implementation, allowing for the calculation of loads they carry during launch.                                                                                                                                                                                                                        | Easy                  | Basic Dynamics                   |
 | Plane Motion Flight Phase                 | Modified dynamics equations.                                                                                                                                                                                                                                                                                               | Hard                  | Advanced Dynamics                |
+| Hybrid/Liquid Motor                       | Derive the equations of motion for non-constant propellant center of mass.                                                                                                                                                                                                                                                 | Hard                  | Advanced Dynamics.               |
 | Custom Aerodynamic Surfaces               | Create a new method in the Rocket Class that allows for custom aerodynamic coefficients such as Drag, Lift and Moment.                                                                                                                                                                                                     | Easy                  | Basic Dynamics and Aerodynamics  |
 | Multibody Dynamics Parachute Flight Phase | Currently, descent under parachute is simulated as a 3 degree of freedom (DOF) system. However, parachutes can and should be modeled separately from the rocket, creating a 6  (DOF) system, 2 for the parachute (spherical constrain) and 3 for the rocket. This can greatly enhance the prediction of the landing point. | Hard                  | Dynamics                         |
 | Graphical User Interface                  | Desktop or Web graphical interface that can be used to run RocketPy in a more user friendly way, possibly also writing the code so the user can learn how to used RocketPy as a module as well.                                                                                                                            | Medium                | None                             |