Improve description of the destination query parameter

Problem/Motivation

There can be error if value for the destination query parameter doesn't begin with '/':
InvalidArgumentException: The user-entered string 'product/1' must begin with a '/', '?', or '#'. in Drupal\Core\Url::fromUserInput() (line 216 of core/lib/Drupal/Core/Url.php).

Steps to reproduce

Use some path without '/' at the beginning of the value for the destination query parameter.
Example: /cart-links/1-2?destination=product/1

Proposed resolution

Improve description on the project's page and readme file.