Make documentation for cusip_cik_exceptions and handle_cusip_cik_exceptions.ipynb

[bdcallen]

@iangow This issue is for making documentation for edgar.cusip_cik_exceptions, explaining what it's for and what its fields are, how I made the initial table, and how one can maintain it using handle_cusip_cik_exceptions.ipynb.

[bdcallen]

@iangow Ok, I've made a first draft of the readme cusip_cik_exceptions.md. I have alse cleaned up handle_cusip_cik_exceptions.ipynb, and added a bunch of markdown cells to it to make it feel more like a notebook with some documentation. The markdown cells further down in the notebook admittedly become a bit terse, which is intentional since the writing needed starts to become a bit repetitive (users can see what's going on by looking at the cells with the code). Let me know what you think. If you're happy with the way things currently are, I think we can close this issue.

[iangow]

Let's put this on the defer list.