Add WattTime docs (#19315)

source/_integrations/watttime.markdown

@@ -0,0 +1,51 @@
+---
+title: WattTime
+description: Instructions on how to set up the WattTime integration within Home Assistant.
+ha_category:
+  - Sensor
+ha_release: 2021.10.0
+ha_iot_class: Cloud Polling
+ha_domain: watttime
+ha_codeowners:
+  - '@bachya'
+ha_config_flow: true
+ha_platforms:
+  - sensor
+---
+
+The `watttime` integration allows users to get real-time emissions data for a latitude/longitude from [WattTime](https://www.watttime.org).
+
+## Registering an Account
+
+WattTime account registration is accomplished [via the REST API itself](https://www.watttime.org/api-documentation/#register-new-user). The simplest way to do this is by using cURL on the command line:
+
+```bash
+curl -X "POST" "https://api2.watttime.org/v2/register" \
+     -H 'Content-Type: application/json; charset=utf-8' \
+     -d $'{
+       "username": "<USERNAME>",
+       "password": "<PASSWORD>",
+       "email": "<EMAIL>",
+       "org": "<ORG>"
+     }'
+```
+
+Note that the `org` value can be anything you like – it doesn't need to represent a real entity.
+
+{% include integrations/config_flow.md %}
+
+## Sensors
+
+<div class='note info'>
+The sensors available to you will depend on the type of WattTime subscription you have. You can reach out to WattTime to upgrade your subscription via [their website](https://www.watttime.org/contact/).
+</div>
+
+| Name                                  | Subscription Level | Meaning                                                                                                     |
+|---------------------------------------|--------------------|-------------------------------------------------------------------------------------------------------------|
+| Marginal Operating Emissions Rate     | Pro                | The currently measured amount of lbs of CO2 per MWh                                                         |
+| Relative Marginal Emissions Intensity | All                | A percentage between the lowest (cleanest) and highest (highest) MOER values observed in the past two weeks |
+
+You can learn more about the data that WattTime collects by viewing these articles:
+
+* https://www.watttime.org/aer/what-is-aer/
+* https://www.watttime.org/aer/how-aer-works/