Document the available ArchLinux package

[damienflament]

Add instructions to install shellspec using the available ArchLinux package from the AUR.
The new section should be used to document installation of packages from other Linux distribution when they will be available.

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   41.39%   41.39%           
=======================================
  Files          81       81           
  Lines        2889     2889           
=======================================
  Hits         1196     1196           
  Misses       1693     1693

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 09b696e...3ac18f3. Read the comment docs.

[ko1nksm]

That's a great job! Thanks!