[dd.conf.sample] Document how MANAGED_EMAIL_DOMAIN

dd.conf.sample

@@ -23,6 +23,15 @@ TITLE="DD"
 TITLE_SHORT="DD"
 DOMAIN=mydomain.com
 # If defined, DD will be managing email for this domain
+# When this variable changes, following commands need to run:
+# # Stop services
+# ./dd-ctl down
+# # Rebuild containers so the variable gets propagated
+# ./dd-ctl build
+# # Restart services with the changed
+# ./dd-ctl up
+# # Reconfigure the SSO provider for the email app
+# ./dd-ctl saml
 #MANAGED_EMAIL_DOMAIN=${DOMAIN}
 LETSENCRYPT_DNS=
 LETSENCRYPT_EMAIL=