Create generic_discovery.markdown

Introduce generic platform discovery with an example

source/developers/generic_discovery.markdown

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+---
+layout: page
+title: "Generic Platform Discovery"
+description: "Using generic platform discovery."
+date: 2016-05-12 22:00 -02:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+---
+
+New controller or hub components often need to add platforms in sub-components (i.e. Lights & Switches) without additional configuration.
+This can be achieved using the `homeassistant.components.discovery.load_platform` method:
+
+```python
+def load_platform(hass, component, platform, info=None, hass_config=None)
+```
+
+From more info on how this works, refer to the [load_platform](https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/components/discovery.py#L78) method.
+
+### {% linkable_title Example %}
+
+Say you need to implement your new MyFlashyHub that controls both Switches & Lights, you can follow these steps:
+
+Configuration required for your new hub component:
+
+```yaml
+myflashyhub:
+   example: setting
+```
+
+The source for your component can be located in your configuration directory for now:
+
+```bash
+~/.homeassistant/custom_components/myflashyhub.py
+~/.homeassistant/custom_components/light/myflashyhub.py
+~/.homeassistant/custom_components/switch/myflashyhub.py
+```
+
+In the hub component `myflashyhub.py` you can call your light and switch components. To pass any non-serializable information to the platforms in the sub-component, you can use a global variable.
+
+```python
+from homeassistant.components.discovery import load_platform
+DOMAIN = 'myflashyhub'
+
+MFH_GLOBAL = None
+
+def setup(hass, config):
+    """Your controller/hub specific code."""
+    
+    global MFH_GLOBAL
+    if MFH_GLOBAL is None:
+        MFH_GLOBAL = SomeObjectToInitialiseGlobal
+    #--- snip ---
+    load_platform(hass, 'light', DOMAIN)
+    load_platform(hass, 'switch', DOMAIN, {'optional': 'arguments'})
+```
+
+Add your custom device specific code to the `setup_platform` method in `light/myflashyhub.py` and `switch/myflashyhub`.
+
+```python
+import homeassistant.components.myflashyhub as myflashyhub
+
+# 'switch' will receive discovery_info={'optional': 'arguments'} 
+# as passed in above. 'light' will receive discovery_info=None
+def setup_platform(hass, config, add_devices, discovery_info=None):
+    """Your switch/light specific code."""
+    # You can now use myflashyhub.MFH_GLOBAL
+```
+
+
+The `load_platform` method allows the platforms to be loaded with the need for any additional platform entries in your `configuration.yaml` file, which normally would have been:
+
+```yaml
+#light:
+#  platform: myflashyhub
+#switch:
+#  platform: myflashyhub
+```
+
+<p class='note '>
+In the past, this was achieved by adding your component to the `DISCOVERY_PLATFORMS` in the target sub-component. Generic discovery through  `load_platform()` allows you to load any sub-component, including custom components, without changing the sub-component.
+</p>