add new Excel row limit of 1,048,576 (i.e. 2^20) rows to data-bulletproofing guide in addition to 2^16 that's already there for previous Excel version
generalize to say that powers of 2 ought to be concerning
Merge pull request #2 from propublica/newexcellimit

add new Excel row limit of 1,048,576 (i.e. 2^20) rows