flatpak installation instructions

[lihaohong6]

Since it is now possible to install yazi through flatpak, the instructions can be added in the installation documentation. I'm not sure where to put it, so I'm opening an issue instead.

The usual content is just a picture with a link to flathub:

<a href='https://flathub.org/apps/io.github.sxyazi.yazi'>
    <img width='120' alt='Download on Flathub' src='https://flathub.org/api/badge?locale=en'/>
</a>

which is rendered as



However, there should be a warning similar to the one on Wezterm's installation page as the flatpak sandbox creates several limitations. The README for the flatpak package lists some limitations and how to overcome them. This README can either be linked or copied over so that users know what to expect.