Docs: add Readme for Raindrop.Bootstrap

Assets/Raindrop/Bootstrap/README.md

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+## Raindrop.Bootstrap
+
+Contains code and scenes that boots up the app.
+
+### Booting up flowchart: 
+
+#### 1. Unity loads the default scene `BootstrapScene.unity`
+
+This scene solely contains `RaindropBootstrapper.cs`, which calls methods to set up logger, static assets folder, etc. 
+
+After all that, `RaindropBootstrapper.cs` starts the LibreMetaverse library, which is dependent on all the above-mentioned.
+
+As a final step, `RaindropBootstrapper.cs` registers `RaindropInstance` to the `ServiceLocator` Singleton.
+
+
+#### 2. Load Game Scene: `MainScene.unity`
+
+When `RaindropBootstrapper.cs` finishes  setting up the environment, it loads the scene `MainScene.unity`. 
+
+`MainScene.unity` contains most of the visible parts of Raindrop Viewer:
+- entire UI 
+- world-space rendered elements/ meshes (map, etc)
+- the overall scene-hierachy for avatar, object, and sims.
+
+Since these elements have dependency on the `LibreMetaverse` library, that's why they are solely in this scene. (I have suffered too many null reference exceptions)

Assets/Raindrop/Bootstrap/README.md.meta

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