Create sensor.cert_expiry.markdown (#2497)

source/_components/sensor.cert_expiry.markdown

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+---
+layout: page
+title: "Certificate Expiry"
+description: "Track HTTPS (SSL) certificate expiry as HomeAssistant sensor."
+date: 2017-04-24 14:14
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Sensor
+ha_release: 0.43
+---
+
+This sensor fetches information from a configured URL and displays the certificate expiry (in days). The `host` and `port` must lead to a working SSL website.
+
+To add the Certificate Expiry sensor to your installation, add these options to `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: cert_expiry
+    host: home-assistant.io
+```
+
+Configuration variables:
+
+- **host** (*Required*): The host FQDN (or IP) to retrieve certificate from
+- **port** array (*Optional*): Port number (default **443**).
+
+<p class='note warning'>
+Make sure that the URL exactly matches your endpoint or resource.
+</p>