doc: document IPv4-only relays

doc/source/iponly.rst

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+.. _iponly:
+
+Hosting without DNS records
+===========================
+
+.. note::
+
+   This option is experimental and might change without notice.
+
+In case you don't have a domain,
+for example in a local network,
+you can run a chatmail relay with only an IPv4 address as well.
+
+To deploy a relay without a domain,
+run ``cmdeploy init`` with only the IPv4 address
+during the :ref:`installation steps <setup>`,
+for example ``cmdeploy init 13.12.23.42``.
+
+Drawbacks
+---------
+
+- your transport encryption will only use self-signed TLS certificates,
+  which are vulnerable against MITM attacks.
+  the chatmail core's end-to-end encryption should suffice in most scenarios though.
+
+- your messages will not be DKIM-signed;
+  experimentally, most chatmail relays accept non-DKIM-signed messages from IPv4-only relays,
+  but some relays might not accept messages from yours.
+