README: Add precisions about minimum requirements

[erdnaxe]

During the installation of the required tools I was surprised by the amount of disk space needed. Maybe the README could be improved by adding:

• An indication of how much disk space we need before attempting the installation.
• An indication of how much memory we need to be able to participate (is 8GiB of RAM enough?).
• A note about the fact that Questa Prime version 10.7 has nothing to do with Quartus ModelSim and the fact that this is the only piece of software that we cannot download by our own.
• An indication about which options we should select in Vivado 2020.1 installer (do we need Vitis?).
• A note about the incompatibility between Vivado 2020.1 and latest Ubuntu LTS and the fact that Vivado 2020.2 solves this bug.

Best regards,

[erdnaxe]

Although these distributions are not officially supported, we could warn that:

• On ArchLinux, vivado AUR package recommends 200GB of disk during installation: https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=vivado#n17
• On NixOS, vivado installer crashes if there is not at least 100GB of disk available in the Nix store with deduplication activated.

Maybe we could advice users to not attempt installation in a virtual machine or on a 256GB SSD?
Is it possible to use Vivado 2019 which is much smaller?