Instructions to install and use calendarium-romanum as a standalone Linux terminal application

[igneus]

Hi, thanks for the effort put into this contribution! Documentation improvements are definitely welcome. I am, however, not going to merge this PR:

• calendarium-romanum is a Ruby gem and is to be installed as such (either gem install ... or specify it as a dependency in a project using Bundler). The way of manual installation you suggest is hacky and I definitely don't encourage anyone to use it.
• Copy the whole folder "calendarium-romanum" (lib/calendarium-romanum) directly into the root "lib" folder on your system (/lib) ... Copy the file "calendariumrom" (bin/calendariumrom) directly into "usr/bin" on your system. -- this wouldn't even work, since /lib is usually not on the Ruby library path list
• the calendariumrom executable currently contains mostly development tools and has rather little value for an end user; I don't advertise it much, since it's planned to extract the calendar querying functionality (currently in the subcommand calendariumrom query) to a separate Ruby gem, depending on calendarium-romanum, and make it more user-friendly and useful

Thank you for your understanding.

[historical-theology]

Strangely enough, it did in fact work flawlessly for me in Manjaro XFCE but not the Gnome DE. I need to reevaluate the matter, clearly. Thank you.

EDIT: I had an erroneous mounting of the rubygem to start, but it was in fact the gem that was running -- not anything that I had copied and pasted. This was a major blunder. Pardon me.