text: Title: Document all the different ways we can interact with reference container objects like dict, set, list
  Body: We've [implemented](https://github.com/nteract/testbook/blob/main/testbook/reference.py#L34-L80) a bunch of container methods which help us do the following with reference objects:
  
  | Internal method| Description| 
  | ------------- |-------------|
  |`__len__` | Find length of a container type object (dict, set, list)|
  |`__iter__` and `__next__`|Iterate through an container|
  |`__getitem__`|Fetch an item by index (or key) from a container|
  |`__setitem__`| Set an item's value in a container|
  |`__contains__`|Find if a container... contains an item|
  
  It would be nice to have examples that show that users can directly interact with reference objects, for example:
  
  An example for finding length would look like:
  
  ```python
  from testbook import testbook
  
  @testbook('notebook.ipynb', execute=True)
  def test_foo(tb):
      my_list = tb.ref('my_list')
      assert len(my_list) == 3 # this internally calls __len__
  ```
  