Added test for model using trait

Added readme for CanBeBoughtTrait
https://github.com/Crinsane/LaravelShoppingcart/issues/427

tests/Fixtures/BuyableProductTrait.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Gloudemans\Tests\Shoppingcart\Fixtures;
+
+use Gloudemans\Shoppingcart\Contracts\Buyable;
+
+class BuyableProductTrait implements Buyable
+{
+    use \Gloudemans\Shoppingcart\CanBeBought;
+
+    /**
+     * @var int|string
+     */
+    private $id;
+
+    /**
+     * @var string
+     */
+    private $name;
+
+    /**
+     * @var float
+     */
+    private $price;
+
+    /**
+     * @var float
+     */
+    private $weight;
+
+    /**
+     * BuyableProduct constructor.
+     *
+     * @param int|string $id
+     * @param string     $name
+     * @param float      $price
+     */
+    public function __construct($id = 1, $name = 'Item name', $price = 10.00, $weight = 0)
+    {
+        $this->id = $id;
+        $this->name = $name;
+        $this->price = $price;
+        $this->weight = $weight;
+    }
+}