add notice about putting imports inside functions (#137)

* add notice about putting imports inside functions

* Fix based on feedback

* notice on custom_components handling of deps

* Update creating_component_deps_and_reqs.md

docs/creating_component_deps_and_reqs.md

@@ -22,6 +22,23 @@ Requirements is a list of strings. Each entry is a `pip` compatible string. For
 REQUIREMENTS = ['pychromecast==0.6.12']
 ```
 
+### Note
+
+Be aware that actual python imports of these dependencies should be done inside functions that use them. This is because Home Assistant installs requirements on demand and so the requirement won't be loaded the first time your component is loaded.
+
+Example:
+
+```python
+REQUIREMENTS = ['pychromecast==0.6.12']
+
+def setup(hass, config):
+    import pychromecast
+    
+    <your code goes here>
+```
+
+### Custom requirements during development & testing
+
 During development of a component, it can be useful to test against different versions of a requirement. This can be done in two steps, using pychromecast as an example:
 
 * `pip install pychromecast==0.6.13 --target ~/.homeassistant/deps`