docs: revamp README with all new features, presets, and commands (#37)

Rewrites README to reflect the current state of the project: 100+ fields
including composite scores (Piotroski, Altman Z, Graham Number) and analyst
data, 14 presets, 9 investor commands, all 18 operators, and asset-specific
field discovery. Replaces the old 7-preset and 2-command documentation.

Author: bearmug