ZTEX: document "--fork"

[solardiz]

Document the introduction of --fork support for ZTEX formats (#4027) in README-ZTEX (including when and how to use it, and when not to).

Mention in README-ZTEX that the way --fork sometimes speeds things up is through asynchronous USB transfers by the forked processes (so that while some transfer others may compute), which puts more pressure on the USB subsystem (as sometimes multiple processes will transfer concurrently) and thus might result in more frequent USB communication errors (exposing hardware, driver, and libusb issues), especially at high forked process counts.