Add api docs for PurchaseUnit class (#33)

* Update purchase-unit.md

Author: mhdcodes

docs/api/purchase-unit.md

@@ -1,4 +1,105 @@
-# Purchase Unit
+# PurchaseUnit
 
-coming soon.
+See https://developer.paypal.com/docs/api/orders/v2/#definition-purchase_unit.
 
+## Methods
+
+### PurchaseUnit::__construct()
+
+Creates a new PurchaseUnit instance.
+
+#### Signature
+
+```php
+public function __construct(AmountBreakdown $amount);
+```
+#### Example
+
+```php
+$amount = AmountBreakdown::of('100.00', 'CAD');
+$purchase_unit = new PurchaseUnit($amount);
+```
+
+### PurchaseUnit::addItems()
+
+Pushes a new item or array of items into the items array of PurchaseUnit.
+
+#### Signature
+
+```php
+public function addItems(array $items): self;
+```
+
+#### Example
+
+```php
+$amount = AmountBreakdown::of('100', 'CAD');
+$purchase_unit = new PurchaseUnit($amount);
+
+$items = array_map(function ($index) {
+    return Item::create("Item $index", '100.00', 'CAD', $index);
+}, [1, 2, 3]);
+
+
+$purchase_unit->addItems($items);
+
+```
+
+### PurchaseUnit::addItem()
+
+Pushes a new item into the items array of PurchaseUnit.
+
+#### Signature
+
+```php
+public function addItem(Item $item): self;
+```
+
+#### Example
+
+```php
+$amount = AmountBreakdown::of('100', 'CAD');
+$purchase_unit = new PurchaseUnit($amount);
+
+$purchase_unit->addItem(Item::create('Item 1', '100.00', 'CAD', 2));
+```
+
+### PurchaseUnit::getItems()
+
+Returns the items array of PurchaseUnit.
+
+#### Signature
+
+```php
+public function getItems(): array;
+```
+
+#### Example
+
+```php
+$amount = AmountBreakdown::of('100', 'CAD');
+$purchase_unit = new PurchaseUnit($amount);
+
+$purchase_unit->addItem(Item::create('Item 1', '100.00', 'CAD', 2));
+$purchase_unit->addItem(Item::create('Item 2', '100.00', 'CAD', 2));
+
+$purchase_unit->getItems();
+```
+### PurchaseUnit::getAmount()
+
+Returns the amount breakdown of PurchaseUnit.
+
+#### Signature
+
+```php
+public function getAmount(): AmountBreakdown;
+```
+
+#### Example
+
+```php
+$amount = AmountBreakdown::of('100', 'CAD');
+$purchase_unit = new PurchaseUnit($amount);
+
+$amount = $purchaseUnit->getAmount();
+```