CHANGELOG.md added

[kholdrex]

This pull request includes several important updates to the irt_ruby project, primarily focusing on adding new features, improving documentation, and updating dependencies.

New Features:

• Missing Data Strategies: Introduced a missing_strategy parameter for Rasch, TwoParameterModel, and ThreeParameterModel to handle nil responses with options :ignore, :treat_as_incorrect, and :treat_as_correct. RSpec tests were updated to cover each strategy.

Documentation Updates:

• Changelog: Added a comprehensive changelog documenting all notable changes in version 0.2.0.
• README: Updated to reflect new missing data strategies, advanced usage, and test instructions. Added examples showcasing how to set missing_strategy for each model.

Dependency Management:

• Gemfile and Gemspec: Removed the matrix gem from Gemfile and added it to the gemspec as a runtime dependency. [1] [2]

Versioning:

• Version Management: Updated gemspec to use a version constant from lib/irt_ruby/version.

These changes enhance the functionality, maintainability, and documentation of the project, ensuring better handling of missing data and providing clearer information for users and developers.