Add inital docs for coinmarketcap (#874)

source/_components/sensor.coinmarketcap.markdown

@@ -0,0 +1,30 @@
+---
+layout: page
+title: "CoinMarketCap"
+description: "Instructions how to integrate CoinMarketCap data within Home Assistant."
+date: 2016-08-31 08:15
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Finance
+ha_release: 0.28
+ha_iot_class: "Cloud Polling"
+---
+
+
+The `coinmarketcap` sensor platform displays various details about a crypto currency provided by [CoinMarketCap](http://coinmarketcap.com/).
+
+To add the CoinMarketCap sensor to your installation, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: coinmarketcap
+    currency: bitcoin
+```
+
+Configuration variables:
+
+- **currency** (*Optional*): The currency to display, eg. `bitcoin`, `litecoin`, `steem`, etc. Default is `bitcoin`.
+