add details about the api key

source/components/sensor.openweathermap.markdown

@@ -12,12 +12,16 @@ footer: true
 <img src='/images/supported_brands/openweathermap.png' class='brand pull-right' />
 The openweathermap platform uses [OpenWeatherMap](http://openweathermap.org/) as a source for meteorological data for your location. 
 
+You need an API key which is free but requires a [registration](http://home.openweathermap.org/users/sign_up).
+
+To add OpenWeatherMap to your installation, add the following to your `configuration.yaml` file:
+
 ```yaml
 # Example configuration.yaml entry
 sensor:
   platform: openweathermap
   api_key: YOUR_API_KEY
-  monitored_variables:
+  monitored_conditions
     - type: 'weather'
     - type: 'temperature'
     - type: 'wind_speed'
@@ -27,4 +31,4 @@ sensor:
     - type: 'rain'
     - type: 'snow'
 ```
-
+Details about the API are available in the (OpenWeatherMap documentation)[http://bugs.openweathermap.org/projects/api/wiki].