Add CHANGELOG.md with project history and features

Author: GSMLG-BOT

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- Initial implementation of HTTP fetch API for Elixir
+- Promise-based asynchronous interface using Elixir Tasks
+- Request cancellation support via AbortController
+- JSON and text response parsing helpers
+- Basic HTTP request/response structs
+- Support for custom headers and request options
+- Content-Type handling for requests with bodies
+- Default timeout of 120 seconds for requests
+
+### Technical Details
+- Uses Erlang's built-in `:httpc` module for HTTP operations
+- Requires Elixir 1.18+ for built-in JSON support
+- Depends on Erlang standard library modules `:inets` and `:httpc`
+- Provides both sync and async operation modes (default: async)
+- Includes comprehensive test suite covering core functionality
+
+## [0.1.0] - 2025-07-30
+
+### Added
+- Initial project setup with Mix
+- Basic project structure and configuration
+- Core HTTP fetch functionality
+- Response and Request struct definitions
+- Promise implementation for async operations
+- AbortController for request cancellation
+- Comprehensive test coverage
+- Documentation and README