Update README.md

[chadbrewbaker]

Added open tax document issues.

[loganwilliams]

Hi Chad, thanks for the suggestion. It's a really interesting topic. If I could make two suggestions that I think would help future contributors to the idea:

• Add a little more context about the issue with a handful of sources
• Are there existing databases of these tax filings, and if so, what are their limitations? (Cost, access rate limitations, OCR, etc?)