added some documentation & explanations

content/sprites/readme.md

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+The sprites are stored in the [Aseprite](https://www.aseprite.org/) format, and you'll need Aseprite to edit them.
+
+A few other notes:
+ - Animations are loaded automatically using Aseprite's `Tag` feature. Each tag is a unique animation
+ - Sprite Origin points use Aseprite's hidden `Slice` feature, which can be used with Shift+C. You can draw slices and give them a pivot point, which we're using to set the origin point of the sprite.