docs: update README.md about installing on Arch Linux

[orhun]

Hey, great project! 🐻

I created the following packages for easy installation on Arch Linux:

• https://aur.archlinux.org/packages/repgrep
• https://aur.archlinux.org/packages/repgrep-git

This PR simply updates README.md to reflect this.

[acheronfail]

Love it, thanks! I just committed to the branch and added a comment saying that you're the maintainer. :slightly_smiling_face:

If you're alright with that, we can merge this.

[orhun]

Works for me!

[acheronfail]

@orhun I just had a look at the PKGBUILD files, I think it might be good to add ripgrep as a dependency?

Otherwise, nice work including all the completion files, etc. 👍

[orhun]

Thanks, added!