Document each summary_type

[lottspot]

Currently there is no documentation for each summary_type which can help frontend engineers interpret the meaning of summary_value.

summary_types which need documentation:

• donation_histogram

[lottspot]

donation_histogram: https://github.com/Lever-age/donation-analysis/wiki/Summary-Types

[lottspot]

keeping this issue open, since we expect that another summary_type will be shortly forthcoming