Added documentation for Yahoo Finance platform (#957)

source/_components/sensor.yahoo_finance.markdown

@@ -0,0 +1,46 @@
+---
+layout: page
+title: "Yahoo Finance"
+description: "Instructions how to setup Yahoo Finance within Home Assistant."
+date: 2016-09-18 21:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: yahoo_finance.png
+ha_category: Finance
+ha_iot_class: "Cloud Polling"
+featured: false
+ha_release: 0.29
+---
+
+The `yahoo_finance` platform uses [Yahoo Finance](https://finance.yahoo.com/) 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: yahoo_finance
+    name: Red Hat Inc.
+    symbol: RHT
+```
+
+Configuration variables:
+
+- **name** (*Optional*): The name of the sensor. If not specified, it defaults to *Yahoo Stock*.
+- **symbol** (*Optional*): The stock market symbol for a given company. If not specified, it defaults to *Yahoo (YHOO)*.
+
+Example configuration:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: yahoo_finance
+    name: Red Hat Inc.
+    symbol: RHT
+
+  - platform: yahoo_finance
+    name: Google
+    symbol: GOOGL
+```