Add tutorial on how to operate with the Unity Plugin

docs/tuto-unity.md

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+# Peace: First steps
+
+Peace is the plugin that brings World to Unity (pun intended). Here is a tutorial
+step by step on how to implement your first real-time endless generating world
+in Unity using World.
+
+## Add the plugin to your project
+
+Go to `Assets > Import Package > Custom Package`.
+Select the package in your filesystem (you have to download it first from the
+[release page](https://github.com/BynaryCobweb/world/releases)).
+A list of files will appear, tick everything if it's not, then confirm your choice.
+
+## Create the scene
+
+- Add a camera. Set the clipping planes to near = 1 and far = 10000. You can add
+a camera controller to be able to move in the scene. If you do not have a camera
+controller, move the camera to (0, 2500, 0) and point it downward.
+- Add empty at (0, 0, 0). You can name it `World`, or anything else that please
+you.
+- Add a RealTimeWorld script to it. <!--
+You can pick the demo you want. Leave empty for default. Available demos are: ... -->
+- Drag the camera to the field `Tracking` of the RealTimeWorld component
+
+![Capture of the RealTimeWorld component](images/realtimeworld-config.png)
+
+- Run!
+
+## Future improvements
+
+At the moment the plugin is not very useful, you can only recreate the demo shipped
+with the project.
+
+In the near future you will be able to pick a json file containing a description
+of your world and explore it in real time in Unity.
+
+<!--
+Features I want to implement:
+ - world autocompletion (interpolation): design your level, put an endlessly
+    generated world around!
+ - during level design:
+    - put an empty somewhere
+    - Attach a world script that specify: radius, maximum resolution
+    - Click 'Regenerate' to add all the assets
+    - Interactive generation: user modification can be returned to world
+-->