Add documentation for new module save_rejected

docs/modules/save_rejected.md

@@ -0,0 +1,55 @@
+---
+title: Save rejected module
+---
+
+# Save rejected module
+
+This module saves all rejected emails to a directory. It is intended for
+debugging, especially when setting up a new server.
+
+The module runs in `idempotent` stage and the MTA is still instructed to reject
+the mail. Hovever Rspamd will save the mail in a directory so it is possible to
+learn it as HAM in case of false positives.
+
+The module is disabled by default. Currently the only other parameter beside
+enabling the module is to set the directory used to save the mail.
+```hcl
+# local.d/save_rejected.conf:
+enabled = true;
+base_path = "/var/lib/rspamd/rejected";
+```
+
+The names of the files created are similar to the names used by Maildir.
+The first part is the unix timestamp, followed by a random string, followed by
+the hostname.
+
+It is necessary to create the directory and set the appropriate permissions:
+```shell
+mkdir -m 0700 /var/lib/rspamd/rejected
+chown _rspamd:_rspamd /var/lib/rspamd/rejected
+```
+
+Furthermore, it might be useful to have a systemd timer for cleanup:
+```ini
+# /etc/systemd/system/clean_rejected.timer
+[Unit]
+Description=Clean old rejected mails
+
+[Timer]
+OnCalendar=hourly
+Persistent=true
+
+[Install]
+WantedBy=timers.target
+```
+
+An associated service that deletes stored emails after one day:
+```ini
+# /etc/systemd/system/clean_rejected.service
+[Unit]
+Description=Clean old rejected mails
+
+[Service]
+User=_rspamd
+ExecStart=/usr/bin/find /var/lib/rspamd/rejected -type f -mmin +1440 -delete
+```