Quick-reference page in docs for statistical calculations

An important purpose of the hyswap package is to provide well-documented, peer-reviewed methods and statistics for our computations. Calculations and statistics are documented within functions, but a quick-reference page describing the main statistical calculations performed (namely percentiles, exceedances, and area-based runoff) would likely be useful. Add to package docs a quick-ref page with sections on:

• Percentile calculations
• Exceedance probability calculations
• Area-based runoff calculations

Text from WaterWatch and NWIS DV/stats service provide a starting point.