I had planned to write a full extensive explanation on how to use doctests, test files and on how to implement testing with exdoc by using mix test
It's however come to my attention that there are already MANY places on the internet that already discuss this.
Since time is valuable I'd rather go ahead with the development.
You can view the changes that occured in 0x05 at the Life Beyond Apocalypse Fossil repository.
Download the full sourcecode here for 0x05 here