Provide example of Uxn interacting with other Uxn cores.

[Ismael-VC]

In src/htm/uxn.htm this is mentioned:

It can interface with up to 16 peripherals, such as screens, controllers, or even other Uxns.

Possible suggestions:

• I think it would be useful to provide an exapmle of this as a link, even if it is an experimental feature.
• We could indicate that this is experimantal only.
• Delete this mention in case this feature won't make it to a release.