Update install-upgrade-asciidoctor.adoc

[alacerda]

Added the Alpine Linux installation (apk) instruction

[mojavelinux]

@alacerda I certainly don't mind add this information about Alpine Linux. However, it seems to me that the Alpine-specific instructions are only related to installing Ruby. After that, it's just the normal gem install approach. I'd rather hold off calling this out until it's possible to install the gem with a single command. Are gems typically packaged for Alpine Linux?

[mojavelinux]

Where I think this change is going to fit is in the planned Ruby environment setup guide (#399).

Once Ruby is installed, the instructions to install Asciidoctor should be the same on all systems (assuming the user is taking the gem install approach).

[mojavelinux]

Actually, it seems like there is now a package on Alpine Linux for Asciidoctor, so this will fit right in next to Debian and Fedora.

$ sudo apk add asciidoctor

[mojavelinux]

Merged a revised version of this PR as 349aca9.

[mojavelinux]

Thanks!