Adding notes on SPF and DKIM

doc/install/SMTP-Email-Setup.md

@@ -13,6 +13,7 @@ To enable email, configure an SMTP email service as follows:
 	         2. Go to [Amazon SES console](https://console.aws.amazon.com/ses) then `SMTP Settings > Create My SMTP Credentials`
 	         3. Copy the `Server Name`, `Port`, `SMTP Username`, and `SMTP Password` for Step 2 below. 
 	         4. From the `Domains` menu set up and verify a new domain, then enable `Generate DKIM Settings` for the domain. 
+	            1. We recommend you set up _[Sender Policy Framework](https://en.wikipedia.org/wiki/Sender_Policy_Framework) (SPF)_ and/or _[Domain Keys Identified Mail](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) (DKIM)_ for your email domain.
 	         5. Choose an sender address like `mattermost@example.com` and click `Send a Test Email` to verify setup is working correctly. 
 
 2.  **Configure SMTP settings**