Update installation instructions for Arch Linux

[orhun]

I have been using igrep extensively for content search for the past couple of weeks and I really enjoy the simplicity. So I moved it to the Arch Linux community repository as an official binary package which means more users will be able to install it easily.

• https://archlinux.org/packages/community/x86_64/igrep/

Also it can now be installed via pacman.

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Cheers!