Enhance Lambda README.md with usage example (#364)

Author: gmorel

README.md

@@ -10,3 +10,24 @@ An API client for Lambda.
 ```cli
 composer require async-aws/lambda
 ```
+
+## Usage
+
+```cli
+export AWS_ACCESS_KEY_ID=AKIAIOSFODNN7EXAMPLE
+export AWS_SECRET_ACCESS_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
+```
+See [docs/authentication.md](https://github.com/async-aws/aws/blob/master/docs/authentication.md) to see how to authenticate against AWS server.
+
+```php
+$client = new LambdaClient([
+    'region' => 'us-east-1',
+]);
+
+$result = $client->invoke([
+  'FunctionName' => 'app-dev-hello_world', // Find it here: https://console.aws.amazon.com/lambda/home?region=us-east-1#/functions
+  'Payload' => '{"name": "async-aws/lambda"}',
+]);
+
+$result->getPayload(); // hello async-aws/lambda
+```