Add guideline for multiple stubs on an object

* `receive_messages` is available in RSpec 3
* Reference: https://github.com/thoughtbot/guides/pull/175#issuecomment-43917785
The challenge of putting everything in one file is that then we are linking to line numbers, and those tend to change when people add to the example files, making the links inaccurate.

In general, I am in favor of including new example files for each example