docs: check DMARC and placement, not only the DKIM keys

[serg-tanichev]

The DKIM verification section recommends dkimvalidator, which confirms the keys and the DNS records. Two things stay open after that check: whether DMARC lines up for the same message, and whether the mail actually reached an inbox rather than a spam folder. On a container sending from a fresh IP, the second one is usually the real problem.

This adds one sentence next to the existing recommendation, which is left as it is. The tool reports inbox, spam or Promotions at Gmail, Outlook, Yahoo and other providers from a single send, and the SPF, DKIM and DMARC verdicts come from the copy that arrived instead of a DNS query. Free, no account.

Disclosure: I maintain the tool.

[bokysan]

Cool, thanks for the update!