Add SMTP indication in example.env

See also https://forum.farmbot.org/t/how-to-configure-smtp-with-gmail/5714

example.env

@@ -79,6 +79,10 @@ SMTP_PORT=587
 # Delete these if you aren't a send grid customer.
 SENDGRID_PASSWORD=Used by FarmBot, Inc
 SENDGRID_USERNAME=Used by FarmBot, Inc
+# If you're using other SMTP server (like Gmail) use this.
+#SMTP_USERNAME=email@gmail.com
+#SMTP_PASSWORD=password
+
 # Used by people who pay for managed database hosting.
 # Most users should delete this.
 DATABASE_URL=postgres://user:password@host:5432/db_name