add mention of good slack notifs module

monitoring/sns-to-slack/README.md

@@ -0,0 +1,13 @@
+Use a community module for this one. I use this one: https://github.com/terraform-aws-modules/terraform-aws-notify-slack
+
+```
+module "slack_webhook" {
+  source  = "terraform-aws-modules/notify-slack/aws"
+  version = "~> 2.0"
+
+  sns_topic_name    = module.sns-notifs.topic_name
+  slack_webhook_url = "https://hooks.slack.com/services/xxx/yyy/zzz"
+  slack_channel     = "ops"
+  slack_username    = "AWS Ops"
+}
+```