Update lets_encrypt.markdown

source/_addons/lets_encrypt.markdown

@@ -9,4 +9,16 @@ sharing: true
 footer: true
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-[Placeholder]
+Setup and manage a [let's encrypt](https://letsencrypt.org/) certificate. The first run will create the certificate and every other run will renew it if they will expire in 30 days. All other run make nothing.
+
+```json
+{
+  "email": "example@example.com",
+  "domains": ["example.com", "mqtt.example.com", "hass.example.com"],
+}
+```
+
+Option variables:
+
+- **email** (*Required*): default empty. Your email for register on let's encrypt with your domains.
+- **domains** (*Required*): default empty list. A list of domains for create/renew the certificate.