docs: add v3 recommended patterns to upgrade guide and link from README

UPGRADE.md

@@ -58,3 +58,108 @@ ### Non-breaking additions
 - **HasCart trait:** Livewire reactive cart state
 - **ShareCartWithInertia:** static class for Inertia middleware integration
 - **Artisan generators:** `cart:install`, `cart:make:coupon`, `cart:make:tax`, `cart:make:shipping`, `cart:make:driver`, `cart:debug`
+
+---
+
+### Recommended v3 patterns
+
+v3 is fully backwards compatible - your existing code will continue to work unchanged. The following patterns are recommended for new code and worth migrating to when you touch existing cart logic.
+
+#### 1. Publish the updated config
+
+Run the installer to get the new `driver` key and storage configuration in your `config/cart.php`:
+
+```bash
+php artisan cart:install
+```
+
+The default driver is `session`, so behaviour is identical to v2 unless you change it.
+
+#### 2. Replace raw CartCondition arrays with first-class objects
+
+Generate typed classes with Artisan instead of constructing arrays inline:
+
+```bash
+php artisan cart:make:coupon TenPercentOff
+php artisan cart:make:tax GstTaxRule
+php artisan cart:make:shipping FlatRateShipping
+```
+
+**Before:**
+
+```php
+$condition = new CartCondition([
+    'name'   => 'SAVE10',
+    'type'   => 'coupon',
+    'target' => 'subtotal',
+    'value'  => '-10%',
+]);
+
+Cart::condition($condition);
+```
+
+**After:**
+
+```php
+// app/Cart/Coupons/TenPercentOff.php
+class TenPercentOff extends Coupon
+{
+    protected string $code = 'SAVE10';
+    protected string $value = '-10%';
+    protected string $target = 'subtotal';
+
+    public function isValid(): bool
+    {
+        return true;
+    }
+}
+
+Cart::coupon(new TenPercentOff);
+Cart::tax(new GstTaxRule);
+Cart::shipping(new FlatRateShipping);
+```
+
+The same pattern applies to `TaxRule` (implement `isApplicable()`) and `ShippingRate` (implement `isApplicable()`).
+
+#### 3. Use Cart::fake() in tests instead of mocking
+
+**Before:**
+
+```php
+// Mocking the session or binding a fake driver manually
+$this->mock(CartDriver::class, function ($mock) {
+    $mock->shouldReceive(...);
+});
+```
+
+**After:**
+
+```php
+$factory = app('cart')->fake();
+
+$factory->withItems(3)->withCondition(new GstTaxRule);
+
+$cart = app('cart');
+$cart->assertCount(3);
+$cart->assertConditionApplied('GST');
+$cart->assertTotal(33.00);
+```
+
+`Cart::fake()` swaps the driver to an in-memory `NullDriver` for the duration of the test - no session, database, or Redis needed.
+
+#### 4. Switch to a persistent driver (optional)
+
+If you want cart state to survive beyond a session, update `config/cart.php`:
+
+```php
+'driver' => 'database', // or 'redis'
+```
+
+For the database driver, publish and run the migration:
+
+```bash
+php artisan cart:install
+php artisan migrate
+```
+
+See the [Storage Drivers](README.md#storage-drivers) section of the README for full configuration options.