Added documentation about ISY994 platform. Also added documentation about creating entities by using the Entity base class.

source/developers/add_new_platform.markdown

@@ -61,3 +61,29 @@ The Discovery component is capable of setting up your components before firing t
 <p class='note warning'>
 This option is currently limited to built-in components.
 </p>
+
+### {% linkable_title Creating Entities %}
+
+Home Assistant will call a function with the following signature to initialize
+your new component. This function must exist in the component module you
+create.
+
+```python
+def setup_platform(hass, config, add_devices, discovery_info=None)
+```
+
+In this function, your component should create the appropriate entities and
+register them with the Home Assistant core. Entities are Home Assistant's
+representation of lights, switches, sensors, etc. It is best practice for all
+new entities to inherit the
+[Entity Abstract Class](https://github.com/balloob/home-assistant/blob/master/homeassistant/helpers/entity.py#L18).
+This abstract class contains logic for integrating most standard features into
+your entities, such as visibility, entity IDs, updates, and many more. That is
+why it is best practice to reference the existing class.
+
+A list of entities can be registered with Home Assitant using the *add_devices*
+function that is provided as an input to *setup_platform*. Once entities are
+registered with with Home Assistant their updates will be provided to the core
+and the core will have control over them. For more information on how Entites
+can be customized, take a look at the [Entity Abstract
+Class](https://github.com/balloob/home-assistant/blob/master/homeassistant/helpers/entity.py#L18).