Add better documentation for the project

Author: stof

README.md

@@ -1,6 +1,8 @@
 Async Amazon Incentives
 =======================
 
+This package provides an unofficial SDK for the [Amazon Incentives API](https://developer.amazon.com/docs/incentives-api/digital-gift-cards.html).
+
 ## Installation
 
 Use [Composer](https://getcomposer.org) to install the library:
@@ -11,6 +13,43 @@ $ composer require incenteev/async-amazon-incentives
 
 ## Usage
 
+```php
+use AsyncAws\Core\Configuration;
+use Incenteev\AsyncAmazonIncentives\AmazonIncentivesClient;
+use Incenteev\AsyncAmazonIncentives\Enum\CurrencyCode;
+use Incenteev\AsyncAmazonIncentives\Exception\SystemTemporarilyUnavailableException;
+use Incenteev\AsyncAmazonIncentives\Region;
+use Incenteev\AsyncAmazonIncentives\ValueObject\MoneyAmount;
+
+// Get your credentials in the Amazon Incentives portal
+$accessKey = '';
+$secretKey = '';
+$partnerId = '';
+
+// Choose the region corresponding to your partnership, with either the sandbox or production one.
+$region = Region::EUROPE_AND_ASIA_SANDBOX;
+
+$client = new AmazonIncentivesClient([
+    Configuration::OPTION_ACCESS_KEY_ID => $accessKey,
+    Configuration::OPTION_SECRET_ACCESS_KEY => $secretKey,
+    Configuration::OPTION_REGION => $region,
+]);
+
+try {
+    $result = $client->createGiftCard([
+        'partnerId' => $partnerId,
+        'creationRequestId' => '', // Create the proper request id
+        'value' => new MoneyAmount(['amount' => 10, 'currencyCode' => CurrencyCode::EUR]),
+    ]);
+
+    $code = $result->getGcClaimCode();
+} catch (SystemTemporarilyUnavailableException $e) {
+    // TODO handle temporary failures according to the Amazon Incentives best practices
+}
+```
+
+> Note: due to the async nature of the project, the exception is not actually thrown by the call to `createGiftCard`
+> but when the Result object gets resolved.
 
 ## License