Installation instructions for Arch Linux users

[firefly-cpp]

NiaPy is now included in AUR. Installation instructions for Arch users should be included in README.

[firefly-cpp]

Thank you @carlosal1015 for packaging NiaPy to AUR. Please commit installation instructions.

[carlosal1015]

Its fine, I am thinking about append the line on README.rst:

To install `NiaPy <https://aur.archlinux.org/packages/python-niapy>`__ on Arch Linux, please use an `AUR helper <https://wiki.archlinux.org/title/AUR_helpers>`__:

.. code:: sh

   $ yay -Syyu python-niapy

Optionally, is recommended to point vote the package for move the package from official repositories.
Also, support Python versions: 3.6.x, 3.7.x, 3.8.x, 3.9.x, 3.10.X, and the slack link is down.

New fork here with the changes.

[firefly-cpp]

Nice. I have no objection to that. @GregaVrbancic, what do you think?

[GregaVrbancic]

I agree with @firefly-cpp. It looks good to me. Additionally, you can also add the installation instructions in README.md.