Added docs about api_version

Author: Nyholm

doc/api_version.md

@@ -0,0 +1,14 @@
+## 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
+the `setApiVersion` function. 
+For example:
+
+```php
+$client = new Github\Client();
+
+echo $client->getApiVersion(); // prints "s3"
+
+$client->setApiVersion("v2");
+```