readme: fix download link, add ArchLinux & macOS

Fix download link to point to v0.3.1 (was: v0.3).

Also, add convenience installation instructions for ArchLinux (via AUR) and macOS (via Homebrew).

Resolves #30, closes #17.

README.md

@@ -14,7 +14,9 @@ by boosting any typical **Linux text-processing utils** such as `grep`, `sort`,
 
 ## Usage
 
-**[Download *up* for Linux](https://github.com/akavel/up/releases/download/v0.3/up)**
+**[Download *up* for Linux](https://github.com/akavel/up/releases/download/v0.3.1/up)**
+  |   [ArchLinux](https://wiki.archlinux.org/index.php/Arch_User_Repository): [`aur/up`](https://aur.archlinux.org/packages/up/)
+  |   macOS: [`brew install up`](https://formulae.brew.sh/formula/up)
   |   [Other OSes](https://github.com/akavel/up/releases)
 
 To start using **up**, redirect any text-emitting command (or pipeline) into it