add BlackArch install steps

[noraj]

• add BlackArch install steps (I added it to BA in add smap-scanner BlackArch/blackarch#4057)
• format Markdown to be CommonMark compliant

[s0md3v]

In my understanding, BlackArch supports installing AUR packages and thus there's no need for this. Let me know if you think otherwise and I'll re-open this PR.

[noraj]

As you want, there a difference between the official BA repository and a third party user repository.

[s0md3v]

I understand but in this case, the AUR package is being managed by the maintainers of the official project not a third party.