README.md Add pipx install method

[superbonaci]

Error in Arch linux:

$ pip install textual-paint
error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try 'pacman -S
    python-xyz', where xyz is the package you are trying to
    install.

    If you wish to install a non-Arch-packaged Python package,
    create a virtual environment using 'python -m venv path/to/venv'.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip.

    If you wish to install a non-Arch packaged Python application,
    it may be easiest to use 'pipx install xyz', which will manage a
    virtual environment for you. Make sure you have python-pipx
    installed via pacman.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

How should be done:

$ sudo pacman -S python-pipx
resolving dependencies...
looking for conflicting packages...

Packages (4) python-argcomplete-2.0.0-2  python-click-8.1.6-1  python-userpath-1.8.0-2  python-pipx-1.2.0-1

Total Download Size:   0.40 MiB
Total Installed Size:  2.17 MiB

:: Proceed with installation? [Y/n] Y
:: Retrieving packages...
 python-click-8.1.6-1-any                                        209.8 KiB   599 KiB/s 00:00 [#########] 100%
 python-pipx-1.2.0-1-any                                         117.8 KiB  1869 KiB/s 00:00 [#########] 100%
 python-argcomplete-2.0.0-2-any                                   57.8 KiB  1090 KiB/s 00:00 [#########] 100%
 python-userpath-1.8.0-2-any                                      22.1 KiB   470 KiB/s 00:00 [#########] 100%
 Total (4/4)                                                     407.3 KiB   590 KiB/s 00:01 [#########] 100%
(4/4) checking keys in keyring                                                               [#########] 100%
(4/4) checking package integrity                                                             [#########] 100%
(4/4) loading package files                                                                  [#########] 100%
(4/4) checking for file conflicts                                                            [#########] 100%
(4/4) checking available disk space                                                          [#########] 100%
:: Processing package changes...
(1/4) installing python-click                                                                [#########] 100%
(2/4) installing python-userpath                                                             [#########] 100%
(3/4) installing python-argcomplete                                                          [#########] 100%
(4/4) installing python-pipx                                                                 [#########] 100%
:: Running post-transaction hooks...
(1/1) Arming ConditionNeedsUpdate...

$ pipx install textual-paint
  installed package textual-paint 0.1.0, installed using Python 3.11.3
  These apps are now globally available
    - textual-paint
done! ✨ 🌟 ✨

[superbonaci]

Thanks!