added resouces on mail server SPF, DKIM, and DMARC

src/mailsrv/docs/Resources.md

@@ -0,0 +1,9 @@
+# Resources
+
+## SPF, DKIM, and DMARC mail protection
+
+- https://www.digitalocean.com/community/tutorials/how-to-use-an-spf-record-to-prevent-spoofing-improve-e-mail-reliability
+- https://www.linuxbabe.com/mail-server/setting-up-dkim-and-spf
+  - says to use `@` for subdomain, but Dyn only accepts `*` as wildcard
+- https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-dkim-with-postfix-on-debian-wheezy
+- https://www.linuxbabe.com/mail-server/create-dmarc-record