Add AUR package reference and instructions

[sankalpgambhir]

Adds the following section to the README, describing the AUR package, copied to be rendered here:

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Arch User Repository

A package for Stainless is available on the Arch User Repository (AUR) for ArchLinux as stainless-git, which follows the latest commit on the main branch. You can use your favorite AUR helper (we've tried yay, see AUR Helpers), or follow the detailed instructions as on the ArchWiki to install the package.

For quick reference, with yay (or other AUR helpers accordingly):

yay -S stainless-git

and manually:

git clone https://aur.archlinux.org/stainless-git.git
cd stainless-git
makepkg -si

The option -s acquires dependencies (java, git, sbt) using pacman, -i installs Stainless system-wide. Omit -i to avoid installing system-wide, and only perform a local build in the directory.

The solver packages z3, cvc4, and cvc5^AUR can be added as optional dependencies, and having at least one is recommended for general use.

Issues with the package build should ideally be reported on the AUR package page itself.

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[sankalpgambhir]

(CI did not fail, was cancelled because it was a waste, having added another commit)

[sankalpgambhir]

Should the line about "having at least one [solver] is recommended" be excluded? Native Z3 should be available anyway.