docs: Add instructions for installing from the AUR

[orhun]

Now available in the AUR!

https://aur.archlinux.org/packages/timr

[kardwen]

I would also consider restructuring the installation instructions in the README a bit. I for example have overlooked the cargo install command which is technically also building timr-tui from source but is located above the "Build from source" section (https://github.com/sectore/timr-tui?tab=readme-ov-file#build-from-source-).

[sectore]

Thanks @orhun!

I would also consider restructuring the installation instructions in the README a bit.

@kardwen Will re-think about it. Feel free to open a PR if its really needed. The README has to be re-structure in any way.