Add hint that functions must be enabled

[Trainmaster]

Here some issues when starting the server:

PHP Warning: pcntl_signal_dispatch() has been disabled for security reasons in /xxx/vendor/amphp/amp/lib/Loop/NativeDriver.php on line 155

PHP Fatal error: Uncaught Error: Failed to register signal handler; Errno: 2; pcntl_signal() has been disabled for security reasons in /xxx/vendor/amphp/amp/lib/Loop/NativeDriver.php:324

I looked at my php.ini and found the following line: disable_functions = pcntl_alarm,pcntl_fork,pcntl_waitpid,pcntl_wait,pcntl_wifexited,pcntl_wifstopped,pcntl_wifsignaled,pcntl_wifcontinued,pcntl_wexitstatus,pcntl_wtermsig,pcntl_wstopsig,pcntl_signal,pcntl_signal_get_handler,pcntl_signal_dispatch,pcntl_get_last_error,pcntl_strerror,pcntl_sigprocmask,pcntl_sigwaitinfo,pcntl_sigtimedwait,pcntl_exec,pcntl_getpriority,pcntl_setpriority,pcntl_async_signals,

It should be mentioned in the README that some (or even all?) of these functions must be enabled.