Document union types, extra value sources and richer output in README

[TomasVotruba]

Updates the README to cover features shipped since it was last written:

• Union types when multiple distinct types reach a parameter (no cap, alphabetically sorted, |null for nullable unions, callable poisons the param)
• Default-value typing without a call site
• Extra argument sources: Foo::class, enum cases, constants/class constants/define(), built-in function returns, method chains on new objects, ?? null nullability
• Docblock cleanup: redundant @param / @param mixed removal, order- and FQN-insensitive matching
• Colored output, per-phase progress bar, added-types category overview
• static:: targets and typed-variable/property/local call-target resolution

Docs only.