Add method to use generate release notes endpoint

Author: GuySartorelli

lib/Github/Api/Repository/Releases.php

@@ -68,6 +68,26 @@ public function show($username, $repository, $id)
         return $this->get('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/releases/'.$id);
     }
 
+    /**
+     * Generate release notes content for a release.
+     *
+     * @param string $username
+     * @param string $repository
+     * @param array  $params
+     *
+     * @throws MissingArgumentException
+     *
+     * @return array
+     */
+    public function generateNotes($username, $repository, array $params)
+    {
+        if (!isset($params['tag_name'])) {
+            throw new MissingArgumentException('tag_name');
+        }
+
+        return $this->post('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/releases/generate-notes', $params);
+    }
+
     /**
      * Create new release in selected repository.
      *