Document support of GraphQL variables

Author: vkublytskyi

CHANGELOG.md

@@ -4,6 +4,10 @@ The change log describes what is "Added", "Removed", "Changed" or "Fixed" betwee
 
 ## 2.6.0 (unreleased)
 
+### Added
+
+- Support for graphql api [variables](https://developer.github.com/v4/guides/forming-calls/#working-with-variables) (#612)
+
 ## 2.5.0
 
 ### Added

doc/graphql.md

@@ -8,3 +8,25 @@ Wraps [GitHub v4 API (GraphQL API)](http://developer.github.com/v4/).
 ```php
 $rateLimits = $client->api('graphql')->execute($query);
 ```
+
+#### Use variables
+
+[Variables](https://developer.github.com/v4/guides/forming-calls/#working-with-variables) allow specifying of requested data without dynamical change of a query on a client side.
+
+```php
+$query = <<<'QUERY'
+query showOrganizationInfo (
+  $organizationLogin: String!
+) {
+  organization(login: $organizationLogin) {
+    name
+    url
+  }
+}
+QUERY;
+$variables = [
+    'organizationLogin' => 'KnpLabs'
+];
+
+$orgInfo = $client->api('graphql')->execute($query, $variables);
+```