Add AirNow Integration (#14502)

Co-authored-by: Franck Nijhof <git@frenck.dev>

source/_integrations/airnow.markdown

@@ -0,0 +1,29 @@
+---
+title: AirNow
+description: Instructions on how to integrate AirNow within Home Assistant.
+ha_category:
+  - Health
+ha_release: 2021.2
+ha_iot_class: Cloud Polling
+ha_config_flow: true
+ha_codeowners:
+  - '@asymworks'
+ha_domain: airnow
+---
+
+The `airnow` integration uses the [AirNow](https://www.airnow.gov/) web service
+as a source for air quality data for your location.
+
+## Setup
+
+To generate an AirNow API key, go to the [AirNow Developer Tools Page](https://docs.airnowapi.org/account/request/) page.
+
+## Configuration
+
+To add AirNow to your installation, go to **Configuration** >> **Integrations** in the UI, click the button with `+` sign and from the list of integrations select **AirNow**. By default, the values will be taken from the Home Assistant configuration.
+
+<div class="note">
+
+The AirNow API allows 500 data updates per hour, but since observations are only updated hourly, the default update rate is set to 2 per hour and should not trigger rate limiting. If you use this API key for other purposes, ensure the total request rate does not exceed 500 per hour.
+
+</div>