Added openexchangerates sensor documentation (#584)

* Create Openexchangerates Markdown

* Update openexchangerates markdown

* Added openexchangerates logo

source/_components/sensor.openexchangerates.markdown

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+---
+layout: page
+title: "Open Exchange Rates"
+description: "Instructions on how to integrate exchange rates from https://openexchangerates.org within Home Assistant."
+date: 2016-06-23 10:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Sensor
+logo: openexchangerates.png
+ha_iot_class: "Local Polling"
+ha_release: 0.23
+---
+
+
+The `openexchangerates` sensor will show you the current exchange rate from [Open Exchange Rates](https://openexchangerates.org) that provides realtime exchange rates for [170 currencies](https://openexchangerates.org/currencies). The free account is limited to only USD as a base currency, allows 1000 requests per month, and updates every hour. 
+
+Obtain your API key [here](https://openexchangerates.org/signup)
+
+To enable this sensor, add the following lines to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  platform: openexchangerates
+  api_key: your_api_key
+  base: USD #optional
+  quote: EUR
+  name: USDEUR #optional
+```
+
+Configuration variables:
+
+- **name** (*Optional*): Name to use in the frontend.
+- **base** (*Optional*): The symbol of the base currency. Defaults to USD 
+- **quote** (*Required*): The symbol of the quote or target currency.