[#38] Add README.md

[brianwalborn]

This pull request significantly enhances the README.md file by providing a comprehensive and user-friendly overview of the Rep Mate application. The updated documentation now includes detailed descriptions of features, technology stack, setup instructions, project structure, and usage guidance, making it much easier for new users and contributors to understand and get started with the project.

Documentation improvements:

• Expanded the introduction to clearly describe Rep Mate as a modern, self-hostable workout tracking application with a focus on privacy and data ownership.
• Added detailed feature sections covering workout tracking, progress history, unit system, cardio support, exercise library, and user profile, with screenshots for visual reference.
• Included a comprehensive tech stack breakdown for frontend, backend, and DevOps, specifying key technologies used in each area.
• Provided step-by-step getting started instructions for both Docker-based deployment and local development, as well as a visual project structure overview.
• Documented key features in detail, described the database schema, and added information on API documentation, contributing, and licensing.