Revise deprecated bash completion instructions for Linux

[wreedb]

As per the FAQ, I revised the instructions for bash completions on Linux, as /etc/bash_completion.d has been considered deprecated for a number of years and it's usage is not recommended by the upstream. I didn't change the Mac instruction line, because I do not own a Mac and cannot verify that.

All that being said; According to the FAQ the best way to actually get the path to the completions directory is to use pkg-config --variable completionsdir bash-completion, though I can't say with any certainty how many users may have have the pkg-config file for bash-completion installed, that might be something distributions like Fedora and Debian would have in a -dev/-devel package. The instructions for a normal user to install them locally are to put them in $XDG_DATA_HOME (default ~/.local/share) in a subdirectory bash-completion/completions/<name>, but I guess that would be something at your discretion (maybe a bit too wordy, etc). But if you do want to use the user-level instructions (which should be portable to Macos, as long as $HOME exists), here is a simple one-line shell command for that, using install since it will create the parent directories:

# Linux
yq completion bash | install -Dm 0644 /dev/stdin ~/.local/share/bash-completion/completions/yq
# Mac
mkdir -p ~/.local/share/bash-completion/completions && yq completion bash > ~/.local/share/bash-completion/completions/yq