Add warning about pcntl web-server use in README

[qozle]

I've been banging my head against why my functions passed to the pool are running synchronously as they are passed in. In my search, I came across spatie's other cool async-esque project, fork (still not entirely sure what the difference is between the two, but I'll just wonder about that, unless someone wants to go off-topic here). While on the git page, I noticed this line in the readme:

❗️ pcntl only works in CLI processes, not in a web context.

ah ha! Hours of cranial abuse have finally led me to an answer. If only this were put non-discretely in the async readme!

A sincere thank you to everyone who has worked on the project =)