Hi,
No, there is no documentation but it's quite easy:
$category = new stdClass();
$category->category_name = "My new category";
$category->category_parent_id = XX;
$categoryClass = hikashop_get('class.category');
$category_id = $categoryClass->save($category);
where XX is the id of the parent category.
Note that if you want to add an image you need also something like that:
$file = new stdClass();
$file->file_ref_id = $category_id;
$file->file_ref_type = 'category';
$file->file_path = 'YY'
$fileClass = hikashop_get('class.file');
$fileClass->save($file);
where YY is the relative path of the image from the upload folder (setting of the HikaShop configuration).