Come up with a system for logging outstanding problems

[DanielSank]

Options:

• Write the problem statement directly into the main document. This seems like a good idea in any case, as it lets people who don't have the book view outstanding problems.

• File an issue identifying outstanding problems. This could be helpful for searching, but is redundant with the open problems in the document.

• Make a branch for outstanding problems with the problem text there. This is sort of contrary to the first option above.