docs: add plugin-focused CONTRIBUTING.md guide (#1533)

CONTRIBUTING.md

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+# Contributing to PrismJS
+
+First off, thank you for considering contributing to PrismJS! It's people like you that make Prism such a great tool. 
+
+This guide will help you understand how Prism works under the hood and how you can add new plugins to extend its functionality.
+
+---
+
+## 1. How Prism Works
+
+At its core, Prism is a lightweight, regex-based syntax highlighter. It works by taking a string of code and a "Grammar" (a collection of regular expressions), and breaking the code down into an array of tokens (like `keyword`, `string`, `function`, etc.). These tokens are then wrapped in HTML `<span>` elements with specific CSS classes, which are styled by Prism's themes.
+
+**The core components are:**
+* **`Prism.core.js`**: The main engine that handles tokenization and hooks.
+* **Grammars**: Objects containing regular expressions that define a language's syntax.
+* **Hooks**: Prism provides a callback architecture (`Prism.hooks`) that allows plugins to modify the core behavior without changing the core code.
+
+---
+
+## 2. Writing a New Plugin
+
+We highly encourage developers to build plugins to extend Prism's functionality beyond basic syntax highlighting (e.g., line numbers, show invisibles, etc.).
+
+Since Prism relies heavily on its hook system, plugins are written by tapping into these hooks rather than modifying the core engine.
+
+1. **Create the directory**: Create a new folder in `plugins/` named `your-plugin-name`.
+2. **Create the files**: Create `prism-your-plugin-name.js` and (if needed) `prism-your-plugin-name.css`.
+3. **Use Hooks**: Hook into Prism's execution lifecycle to modify the DOM or the token stream.
+```javascript
+   Prism.hooks.add('complete', function (env) {
+       // Your plugin logic here
+       // 'env' contains the code, grammar, language, and the DOM element
+   });
+Register the Plugin: Make sure to add your new plugin to components.json or the relevant build configuration so the build system recognizes it and can generate the necessary bundles.
+
+3. Submitting your Pull Request
+Fork the repository and create your branch from master.
+
+Run npm install to get everything set up.
+
+Make your plugin additions.
+
+Run npm test and ensure all tests pass (Prism uses standard Mocha tests).
+
+Commit your changes with a clear and descriptive commit message.
+
+Open a Pull Request! Please include a summary of your changes and reference the issue number if applicable.