Add coinbase component and sensor (#4168)

* coinbase. pr 11036

* Update coinbase.markdown

* exchange rate sensors

* Update coinbase.markdown

* Update sensor.coinbase.markdown

* Update coinbase.markdown

source/_components/coinbase.markdown

@@ -0,0 +1,47 @@
+---
+layout: page
+title: "coinbase"
+description: "Instructions for how to add Coinbase sensors to Home Assistant."
+date: 2017-12-08 17:54
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: coinbase.png
+ha_release: 0.61
+ha_iot_class: "Cloud Polling"
+---
+
+
+The `coinbase` component lets you access account balances and exchange rates from [coinbase](https://coinbase.com).
+
+You will need to obtain an API key from coinbase's [developer site](https://www.coinbase.com/settings/api) to use this component. You need to give read access to `wallet:accounts` in order for the component to access relevant data. 
+
+To set it up, add the following information to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+coinbase:
+  api_key: asdfghjklqwertyuiopzxcvbnm 
+  api_secret: nkjnewncfghjklqwertyuiopzxcvbnm 
+  exchange_rate_currencies:
+    - BTC
+    - ETH
+    - LTC
+=======
+```
+
+{% configuration %}
+api_key:
+  description: Your API key to access coinbase.
+  required: true
+  type: string
+api_secret:
+  description: Your API secret to access coinbase.
+  required: true
+  type: string
+exchange_rate_currencies:
+  description: List of currencies to create exchange rate sensors for.
+  required: false
+  type: list
+{% endconfiguration %}

source/_components/sensor.coinbase.markdown

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+---
+layout: page
+title: "Coinbase Sensor"
+description: "Instructions how to setup Coinbase sensors within Home Assistant."
+date: 2017-12-08 18:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: coinbase.png
+ha_category: Sensor
+ha_release: 0.61
+---
+
+To get your coinbase sensors working with Home Assistant, follow the instructions for the general [Coinbase component](/components/coinbase/).