You can use an array in the select() to define more columns and you can use the DB::raw() there with aliasing it to followers. Should look like this:
$query = DB::table('category_issue')
->select(array('issues.*', DB::raw('COUNT(issue_subscriptions.issue_id) as followers')))
->where('category_id', '=', 1)
->join('issues', 'category_issue.issue_id', '=', 'issues.id')
->left_join('issue_subscriptions', 'issues.id', '=', 'issue_subscriptions.issue_id')
->group_by('issues.id')
->order_by('followers', 'desc')
->get();