Add Alpha Vantage sensor docs (home-assistant#4122)

source/_components/sensor.alpha_vantage.markdown

@@ -0,0 +1,53 @@
+---
+layout: page
+title: "Alpha Vantage"
+description: "Instructions how to setup Alpha Vantage within Home Assistant."
+date: 2017-12-02 12:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: alpha_vantage.png
+ha_category: Finance
+ha_iot_class: "Cloud Polling"
+featured: false
+ha_release: "0.60"
+---
+
+The `alpha_vantage` sensor platform uses [Alpha Vantage](https://www.alphavantage.co) to monitor the stock market.
+
+To enable the `yahoo_finance` platform, add the following lines to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: alpha_vantage
+    api_key: YOUR_API_KEY
+```
+
+{% configuration %}
+api_key:
+  description: "The API Key from [Alpha Vantage](https://www.alphavantage.co)."
+  required: true
+  type: string
+symbols:
+  description: List of stock market symbols for given companies.
+  required: false
+  type: string, list
+  default: GOOGL
+{% endconfiguration %}
+
+## {% linkable_title Examples %}
+
+In this section you find some real life examples of how to use this sensor.
+
+### {% linkable_title Red Hat and Google %}
+
+```yaml
+sensor:
+  - platform: yahoo_finance
+    symbols:
+      - RHT
+      - GOOGL
+```
+

source/_components/sensor.yahoo_finance.markdown

@@ -15,7 +15,7 @@ ha_release: 0.29
 ---
 
 <p class='note warning'>
-  This sensor doesn't work anymore as [Yahoo!](https://yahoo.uservoice.com/forums/382977-finance/suggestions/32103877-yahoo-stock-quote-api-please-bring-back) decommissioned the service in early November 2017.
+  This sensor doesn't work anymore as [Yahoo!](https://yahoo.uservoice.com/forums/382977-finance/suggestions/32103877-yahoo-stock-quote-api-please-bring-back) decommissioned the service in early November 2017. A repleacement is the [`alpha_vantage` sensor](/components/sensor.alpha_vantage/).
 </p>
 
 The `yahoo_finance` platform uses [Yahoo Finance](https://finance.yahoo.com/) to monitor the stock market.