Add some documentations.

DOC/README.postfix_does_not_start

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+# - After upgrade from jessie to strech systemd seems to "ignore" the Postfix startup.
+# -
+# - Also "systemctl start postfix" does not work.
+# -
+# - I found out that I simply have to reload systemd with
+# -    systemctl daemon-reload
+# -
+# - But after reboot, again no psotfix daemon is running
+# -
+
+# - Finally the following simple command fixed this problem
+# -    systemctl enable postfix@-
+# -
+# - After rebooting, on some systems postfix starts up fine.
+