chore: add terraform configuration for CNAME-type DNS records.

Part of #1000

[skip ci]

Author: php-coder

infra/terraform/README.md

@@ -21,6 +21,7 @@
   $ terraform import digitalocean_record.ns3 my-stamps.ru,<id>
   $ terraform import digitalocean_record.mx1 my-stamps.ru,<id>
   $ terraform import digitalocean_record.mx2 my-stamps.ru,<id>
+  $ terraform import digitalocean_record.email my-stamps.ru,<id>
   ```
   The ids can be obtained by API:
   - https://developers.digitalocean.com/documentation/v2/#list-all-droplets

infra/terraform/my-stamps.tf

@@ -68,3 +68,9 @@ resource "digitalocean_record" "mx2" {
   value    = "mxb.mailgun.org."
   priority = 10
 }
+resource "digitalocean_record" "email" {
+  domain   = digitalocean_domain.site.name
+  type   = "CNAME"
+  name   = "email"
+  value  = "mailgun.org."
+}