[Documentation] Could a standalone example be added for working with tab-completion? #862

Hey there,

Could a standalone example be added to showcase how to use reline with regards
to tab completion? For instance in a REPL:

cd /home/fo<PRESS_TAB_HERE>

I can probably look at readline, but I think it would be better to have the documentation
within the reline project itself here. Finding documentation in ruby is getting increasingly
difficult; I can barely find anything anymore with google search and there also does not
seem to be a plethora of discussions about ruby anymore. We seem to lose documentation
rapidly right now (excluding Burdette Lamar who is doing a heroic job to counter this).
This loss of easy as-is documentation for ruby-project should be countered in general.