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

Part of #1000

[skip ci]

Author: php-coder

infra/terraform/README.md

@@ -19,6 +19,8 @@
   $ terraform import digitalocean_record.ns1 my-stamps.ru,<id>
   $ terraform import digitalocean_record.ns2 my-stamps.ru,<id>
   $ 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>
   ```
   The ids can be obtained by API:
   - https://developers.digitalocean.com/documentation/v2/#list-all-droplets

infra/terraform/my-stamps.tf

@@ -54,3 +54,17 @@ resource "digitalocean_record" "ns3" {
   name   = "@" # <-- to match the current settings. It's better to use "ns3" instead
   value  = "ns3.digitalocean.com."
 }
+resource "digitalocean_record" "mx1" {
+  domain   = digitalocean_domain.site.name
+  type     = "MX"
+  name     = "@" # <-- to match the current settings. It's better to use "mx1" instead
+  value    = "mxa.mailgun.org."
+  priority = 10
+}
+resource "digitalocean_record" "mx2" {
+  domain   = digitalocean_domain.site.name
+  type     = "MX"
+  name     = "@" # <-- to match the current settings. It's better to use "mx2" instead
+  value    = "mxb.mailgun.org."
+  priority = 10
+}