Update index.md

Author: Snorfield

docs/Resources/index.md

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 This page provides a list of external resources for learning and extra enrichment.
 
-- [Godot Shader Docs](https://docs.godotengine.org/en/stable/tutorials/shaders/index.html)
+- [Godot Shader Docs](https://docs.godotengine.org/en/stable/tutorials/shaders/index.html) Documentation for Godot's shader system
+- [ShaderToy](https://www.shadertoy.com/) Web-based fragment shader playground
+- [Iris Shader Docs](https://shaders.properties/) Documentation for writing Minecraft shaderpacks using [Iris](https://modrinth.com/mod/iris)
+- [The Book Of Shaders](https://thebookofshaders.com/) Step-by-step fragment shader guide, covers a lot of different things with great skill