Explain how contributions are aggregated

[jeremiak]

Michael Snell from Sacramento shows up twice as a donor to Valenzuela, which is weird since we say we aggregate the contributions.

The reason is that we use the zip code and the contributor name to determine uniqueness and in this case Michael Snell used two different zip codes: 95815 and 95818.

Though this isn't necessarily nefarious (Snell could have moved since the contributions are all from the same zip code in the same year) we should explain why a person might be seeing this or show the zip code or both.

The value of using the zip code is in case there are people with very common names donating to the same candidate, but maybe that doesn't happen and isn't worth the confusion.

Should be part of #28/#30