add a readme file

README.md

@@ -0,0 +1,17 @@
+## Apache SpamAssassin in docker
+
+- SpamAssassin is listening over a `783/tcp` port.
+- `chmod 755 /etc/postfix /etc/postfix/main.cf /etc/postfix/dynamicmaps.cf`
+
+- `/etc/postfix/master.cf` example:
+
+> `spamassassin` host should resolve the IP of this container.
+
+```
+smtp      inet  n       -       n       -       -       smtpd
+  -o content_filter=spamassassin
+submission inet n       -       n       -       -       smtpd
+  -o content_filter=spamassassin
+spamassassin      unix  -       n       n       -       -       pipe
+  user=nobody argv=/usr/bin/spamc -d spamassassin -p 783 -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}
+```