Merge pull request KnpLabs#418 from Nyholm/api-version

Added docs about api_version

Author: cursedcoder

doc/api_version.md

@@ -0,0 +1,13 @@
+## Api version
+[Back to the navigation](README.md)
+
+If you want to change the API version from its default ("v3") you may do that with a constructor argument.
+For example:
+
+```php
+$client = new Github\Client();
+echo $client->getApiVersion(); // prints "v3"
+
+$client = new Github\Client($httpClient, 'v2');
+echo $client->getApiVersion(); // prints "v2"
+```