Add some info on DMARC

I ran a check on http://mail-tester.com and found out I was missing a header to be set for DMARC. SInce I missed it, maybe someone else missed it as well, so maybe it could go into the README.

README.md

@@ -203,6 +203,8 @@ To ensure your emails pass DKIM checks you need to add a `txt` record. The name
 
     v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDKKAQfMwKVx+oJripQI+Ag4uTwYnsXKjgBGtl7Tk6UMTUwhMqnitqbR/ZQEZjcNolTkNDtyKZY2Z6LqvM4KsrITpiMbkV1eX6GKczT8Lws5KXn+6BHCKULGdireTAUr3Id7mtjLrbi/E3248Pq0Zs39hkDxsDcve12WccjafJVwIDAQAB
 
+For DMARC you'll also need to add a `txt` record. The name field should be `_dmarc.EXAMPLE.COM` and the value should be `v=DMARC1; p=none`. More info on DMARC can be found [here](https://dmarc.org)
+
 Set up SPF and reverse DNS [as per this post](http://sealedabstract.com/code/nsa-proof-your-e-mail-in-2-hours/). Make sure to validate that it’s all working, for example by sending an email to <a href="mailto:check-auth@verifier.port25.com">check-auth@verifier.port25.com</a> and reviewing the report that will be emailed back to you.
 
 ### 7. Miscellaneous Configuration