Release 0.2.0: override-by-default readers, mutable default fix, README and changelog polish

[kdonovan]

Summary

This PR prepares and documents release 0.2.0 of declarative_initialization.

Behavior changes

• Override by default: When a method with the same name as an init-arg already exists (on the class or an ancestor), the gem now always defines the reader and overrides that method, so foo consistently returns the init-arg value. Previously it skipped defining the reader and warned; callers had to use @foo. This is a breaking change for code that relied on the old “skip and warn” behavior.
• Mutable defaults: Default values for Array, Hash, Set, and String are now duplicated per instance when the caller omits that keyword (shallow copy). That prevents accidental cross-instance mutation when using defaults like items: []. Caller-provided values are not duplicated.

Internal / UX

• Validation and override-warning logic moved into a new DeclarativeInitialization::Internal module (module functions only; no injection into user classes). Override warnings are optional: they run in Rails dev/test or when the logger level is DEBUG. No warning on Rails reload when the method was originally defined by the gem, and no warning when a subclass re-declares an attribute from a parent’s initialize_with.
• Reader setup now tracks declared readers (including from ancestors) so reload and inheritance cases are handled without spurious warnings.
• README rewritten for a RubyGems-style, quick-scan flow: what it does, when to use it, install, quick start, usage, and behavior notes (including the mutable-default behavior and how to get private readers).
• CHANGELOG reorganized with clear Added / Changed / Breaking / Fixed sections for production releases.