Add Double Dispatch pattern

[sadukie]

Put in DDD and link to Design Patterns? Put in Design Patterns and link to DDD? Make sure there are cross-links.

See:

• https://ardalis.com/double-dispatch-in-c-and-ddd/#sq_hjg1ro9bdi

See also:

• https://www.baeldung.com/ddd-double-dispatch
• https://wiki.c2.com/?DoubleDispatch