Update README.md with local development and testing instructions

[devin-ai-integration]

Update README.md with local development and testing instructions

Added comprehensive instructions for local development and testing of the Liquid Labs CLI, including:

• Setting up a development environment
• Multiple methods for running the CLI locally (through Python module, uv run, or after installation)
• Commands for running tests with different options
• Instructions for code quality checks using black and flake8

These additions will help developers understand how to work with the codebase more effectively.

Link to Devin run: https://app.devin.ai/sessions/f366bac3fcbf42d7a937b74a2842e730
Requested by: liren@liquid.ai

[devin-ai-integration]

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