readme: note --csv-headers exits

[vadcx]

This is now in line with --help description. I thought that --csv-header would print the header AND the results when combined with --csv.

I didn't add myself to the page authors or update the date. Note that you didn't update the date in the first line in the previous readme change too: https://github.com/sivel/speedtest-cli/commit/3109fcf407a948f1a867da122267cfe9ece7150f I don't know if this is required.