chore: add migration guide for v3 (#1065)

Author: baseballyama

docs/migration.md

@@ -62,3 +62,99 @@ Therefore, you need to replace the package name, and the presets, rules, and set
     +    "svelte": { ... }
       },
     ```
+
+## From `eslint-plugin-svelte` v2 To v3
+
+This section explains the necessary changes when upgrading from `eslint-plugin-svelte` v2 to v3.  
+v3 includes **support for ESLint Flat Config only**, **changes to the recommended rule set**, and other breaking changes.
+
+---
+
+## Breaking Changes
+
+### 1. **Minimum Node.js Version Requirement**
+
+v3 requires **one of the following Node.js versions**:
+
+- `^18.20.4`
+- `^20.18.0`
+- `>=22.10.0`
+
+### 2. **ESLint Flat Config Only**
+
+- `.eslintrc.js` and `.eslintrc.json` are **no longer supported**.
+- You must use **Flat Config (`eslint.config.js`)**.
+- See [README](README.md) for more details.
+
+### 3. **ESLint Version Requirement**
+
+- v3 requires **ESLint 8.57.1 or later** (including v9.x).
+
+### 4. **Changes to Recommended Rule Set**
+
+- The following rules are now included in `recommended`:
+  - `svelte/infinite-reactive-loop`
+  - `svelte/no-dom-manipulating`
+  - `svelte/no-dupe-on-directives`
+  - `svelte/no-reactive-reassign`
+  - `svelte/require-event-dispatcher-types`
+  - Many others (See [Changelog](https://github.com/sveltejs/eslint-plugin-svelte/releases/tag/eslint-plugin-svelte%403.0.0-next.16) for more details)
+- `svelte/valid-compile` has been **removed** from `recommended`.
+
+### 5. **Deprecated Rules**
+
+- `svelte/no-dynamic-slot-name` is **deprecated**.
+- `svelte/no-goto-without-base` is **deprecated** and replaced with `svelte/no-navigation-without-base`.
+
+---
+
+## New Features & Improvements
+
+### 1. **Support for Svelte 5**
+
+- Rules such as `no-not-function-handler` and `valid-prop-names-in-kit-pages` now support **Svelte 5**.
+
+### 2. **New Rules**
+
+- `consistent-selector-style`: Enforce consistent selector styles.
+- `no-useless-children-snippet`: Prevent unnecessary `{@children}` usage.
+- `prefer-const`: Prefer `const` over `let`.
+- `no-navigation-without-base`: Ensure proper usage of `goto()`.
+
+---
+
+## Migration Steps
+
+### 1. **Upgrade Package**
+
+```sh
+npm install eslint-plugin-svelte@3
+```
+
+### 2. **Check Node.js Version**
+
+```sh
+node -v
+```
+
+- Ensure it is `v18.20.4` or later.
+
+### 3. **Check ESLint Version**
+
+```sh
+npx eslint -v
+```
+
+- Ensure it is `8.57.1` or later (or `9.x`).
+
+### 4. **Use Flat Config (`eslint.config.js`)**
+
+- Remove `.eslintrc.js` and migrate to `eslint.config.js`.
+
+### 5. **Apply the Updated Recommended Rules**
+
+- If using `recommended`, check the impact of newly added rules.
+
+### 6. **Replace Deprecated Rules**
+
+- Replace `no-goto-without-base` with `no-navigation-without-base`.