Docs: make some prerequisites pre-installation

[callmebob2016]

I was trying to do it myself, but looks like I cannot create a branch / pull request 🤷🏼‍♂️

The below 2 points form Post Installation Guide should be moved to Prerequisites in README.md

• Make sure that UEFI/secure boot is disabled!
• When you are logging into your session you are using X server (X11) and not Wayland.

Those should be clearly visible as prerequisites - I don't want to download and install the package just to learn that I need to be using X11 or I need to disable my secure boot. This should be clear from the start.

So my suggestion is to remove the 2 points from docs/post-install-guide.md and do the below in README.md:

Installation

Prerequisites

• Make sure that UEFI/secure boot is disabled!
• When you are logging into your session you are using X server (X11) and not Wayland.

Installation process

1. Download the repo, cd in the directory and run the shell script with sudo:

   git clone https://github.com/AdnanHodzic/displaylink-debian.git
   cd displaylink-debian
   sudo ./displaylink-debian.sh

   [...]