I have put together the new little football-cat gem 
that lets you concatenate match files (for clubs or national teams)
in the one-line, one-match & one-file, one-season Football.CSV format
into one PLUS (auto-)adding the missing league and season headers / columns
inferred from the file's basename (e.g. es.1.csv => ES1)
and the file's directory (e.g. /2020-21/ => 2020/21), for example.
Happy data wrangling with ruby. Enjoy the beautiful game.