document how to get a list of products for matching with --q-product

[ryran]

Either in help page or README. Example:

rhsecapi --q-after 2019-01-01 -x -j |
  jq -r '.[].affected_release[]?.product_name?, .[].affected_release?.product_name?' |
    sort -u

Full list as of 2019/07/14: https://gist.github.com/ryran/112cb263f3824b023d72911a21013adf