Merge pull request #184 from edbeeching/add_working_with_c_sharp_doc

[WIP] Add working with c sharp doc

docs/WORKING_WITH_CSHARP.md

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+# Working with C#
+
+While the Godot RL Agents Godot plugin currently natively supports only working with gdscript and this option is recommended, 
+it is possible to use C# for your projects as well. The process is slightly more complicated, so some understanding of
+how the gdscript variant of the plugin works is recommended.
+
+We recommended completing the [custom env](https://github.com/edbeeching/godot_rl_agents/blob/main/docs/CUSTOM_ENV.md) tutorial first using
+gdscript to get an idea of the usual process first.
+
+We have prepared a simple example that comes in 3 variants:
+- [GDScript](https://github.com/edbeeching/godot_rl_agents_examples/tree/main/examples/TestExamples/SimpleReachGoal/GDScript) (The entire game is written in gdscript)
+- [CSharp](https://github.com/edbeeching/godot_rl_agents_examples/tree/main/examples/TestExamples/SimpleReachGoal/CSharp) (Most of the game is written in C#, except for the extended AIController which is written in GDScript)
+- [CSharpAll](https://github.com/edbeeching/godot_rl_agents_examples/tree/main/examples/TestExamples/SimpleReachGoal/CSharpAll) (The entire game is written in C#, and interfacing with the plugin is done through an AIController also  written in C#)
+
+### CSharp
+
+In this approach, we extend the AIController using gdscript as we cannot directly extend from it using C#, but we still write the rest of the game in C# (e.g. Player and other classes). 
+
+You can see the [extended AIController here](https://github.com/edbeeching/godot_rl_agents_examples/blob/main/examples/TestExamples/SimpleReachGoal/CSharp/scenes/player/player_ai_controller.gd). The code is very similar to the gdscript version [extended AI Controller](https://github.com/edbeeching/godot_rl_agents_examples/blob/main/examples/TestExamples/SimpleReachGoal/GDScript/scenes/player/player_ai_controller.gd), except that we don't get code completion in gdscript for the C# classes. It is slightly more complex to access/modify the gdscript AIController properties from the C# Player class, and you can see how that is done in the [Player script](https://github.com/edbeeching/godot_rl_agents_examples/blob/main/examples/TestExamples/SimpleReachGoal/CSharp/scenes/player/Player.cs).
+
+The main advantage of this approach is that it doesn't require any modifications to the plugin, or writing an AIController wrapper (see [CSharpAll](#csharpall) for that approach). It's also easy to access data from [sensors](https://github.com/edbeeching/godot_rl_agents/blob/main/docs/NODE_REFERENCE.md#sensors) included in the plugin which are written in gdscript. The disadvantage is that it uses [cross-language scripting](https://docs.godotengine.org/en/stable/tutorials/scripting/cross_language_scripting.html) when interfacing between the AIController and the Player script and/or the rest of the env code written in C#.
+
+### CSharpAll
+
+As mentioned the _CSharpAll_ variant of this example has the entire game written in C#, aiming to provide a more 
+consistent experience for C# developers. The main advantage of this approach is that interfacing with the AIController 
+no longer assumes [cross-language scripting](https://docs.godotengine.org/en/stable/tutorials/scripting/cross_language_scripting.html) between C# and GDScript.
+
+This approach is slightly more complicated to set up than the _CSharp_ variant, as it requires a custom C# wrapper for 
+the `ai_controller_3d.gd` node. We provide this wrapper called `AIControllerSharp3D.cs` [here](https://github.com/edbeeching/godot_rl_agents_examples/blob/main/examples/TestExamples/SimpleReachGoal/CSharpAll/scenes/player/AIControllerSharp3D.cs),
+but it should be noted that this wrapper may not be kept up to date with the GDScript version, as such it may need small
+changes in the future to work.
+
+#### Using the C# AIController
+
+As we did in the other examples, we need to inherit from the AIController node, only now we will inherit from the 
+provided C# wrapper. Upon inheriting, you will find the C# equivalent of the GDScript AIController functions that you 
+need to add logic to, see this section of [custom env](https://github.com/edbeeching/godot_rl_agents/blob/main/docs/CUSTOM_ENV.md#adding-the-ai-controller) tutorial for more details.
+
+As mentioned before, by writing the AIController in C# we eliminate [cross-language scripting](https://docs.godotengine.org/en/stable/tutorials/scripting/cross_language_scripting.html) between the 'player'
+C# script and the AIController script, but this does mean that interfacing with other GDScript nodes, such as the 
+sensors provided by the plugin, means a minimal cross-language scripting is still required. This example also showcases
+how to inferface with these components (see [PlayerAIController.cs](https://github.com/edbeeching/godot_rl_agents_examples/blob/main/examples/TestExamples/SimpleReachGoal/CSharpAll/scenes/player/PlayerAIController.cs)).
+
+#### Modifying the AIController 3D wrapper to 2D version
+
+In case you want to use the 2D version of the AIController, you will need to modify the provided 
+`AIControllerSharp3D.cs` in a few minor ways:
+- Change the class name to `AIControllerSharp2D` and the base class to `Node2D`
+```diff
+- public abstract partial class AIControllerSharp3D : Node3D
++ public abstract partial class AIControllerSharp2D : Node2D
+```
+- Change the `_player` field to be of type `Node2D` instead of `Node3D`
+```diff
+- public Node3D _player;
++ public Node2D _player;
+```