Add some documentation for running processes

[nesk]

I've just noticed Psy Shell was acting strange when running processes. See this example:

>>> $p = proc_open('nc -l', [], $pipes)
=> process resource #190
>>> proc_get_status($p)['running']
=> false

I'm starting a new process with the listening mode of netcat here, which runs indefinitely. The running status property returns false, although the netcat process is still running. It seems like PHP can't see it running.

After searching for a while, I found the solution in #198. There is a usePcntl option, I set it to false and now everything works fine, the running status property now returns true.

It would be worth talking about this kind of issue in the documentation. 🙂

[bobthecow]

Added to the Troubleshooting documentation: https://github.com/bobthecow/psysh/wiki/Troubleshooting