Update to centos8.md community guide

[Arkanuki]

I added the php-process package containing the posix php module required by Composer to the dependencies list.

I was blocked because Composer was throwing Root composer.json requires PHP extension ext-posix * but it is missing from your system. Install or enable PHP's posix extension.

The posix extension needed is part of the php-process package. Adding that package fixed the problem.

[rowansc1]

It's probably fine to implement, I'm going to replicate the issue first though.

[rikodanz31-eng]

bug wa / kenon wa

[rikodanz31-eng]