Create a basic structure guide

CONTRIBUTING.md

@@ -25,6 +25,10 @@ There are several ways you can contribute to Pumpkin:
   Ensure your code adheres to the project's coding style guidelines (if any).
   Write clear and concise commit messages that describe your changes.
 
+### Project Structure
+
+Before contributing, it would be helpful to get to know the project structure, for further information, visit [STRUCTURE.md](STRUCTURE.md)
+
 ### Additional Information
 
 We encourage you to comment on existing issues and pull requests to share your thoughts and provide feedback.

STRUCTURE.md

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+# Project Structure
+
+## Overview
+
+Pumpkin is split into multiple crates, thus having a set project structure between contributors is essential.
+
+## Pumpkin-Core
+
+The core crate has some special rules that only apply to it:
+
+- It may not depend on any other pumpkin crate
+- There may not be any files directly under src/, except for the mod.rs file (this is to help with organisation)
+
+## Other crate rules
+
+- [`pumpkin-protocol`](/pumpkin-protocol/) - contains definitions for packet types **and** their serialization (be it through serde, or manually implementing `ClientPacket`/`ServerPacket`), only the `pumpkin` crate may depend on this
+- `pumpkin-macros` - similarly to `pumpkin-core`, it may not depend on any other pumpkin crate