Document Let's Encrypt certificate expiration notice for domain

## what
* "Let's Encrypt certificate expiration notice for domain "
* We start getting these email notifications for "Unlimited Staging environments" where we have one environment for each PR or branch (e.g. `api.cp-312.us-west-2.staging.cloudposse.co`)

```
Your certificate (or certificates) for the names listed below will expire in 20 days (on 12 Dec 18 14:01 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.

We recommend renewing certificates automatically when they have a third of their
total lifetime left. For Let's Encrypt's current 90-day certificates, that means
renewing 30 days before expiration. See
```

## why
* These short lived environments leave TLS certs dangling.
* Its safe to ignore them for these environments