Copy edit on README

[timsneath]

This pull request includes several updates to the README.md file for the Swift extension for Visual Studio Code. The changes enhance the clarity and completeness of the documentation, ensuring users have a better understanding of the extension's features and setup requirements.

Key updates to documentation:

• Improved the introduction to highlight the seamless experience for developing Swift applications on all supported platforms.
• Updated the description of dependencies and installation steps to be more detailed and user-friendly.
• Clarified the commands available in the extension, specifying those that are exclusive to macOS and refining the descriptions for better understanding. [1] [2]
• Enhanced the explanation of debugging features, including the automatic generation of build tasks and launch configurations, and the handling of lldb for Swift debugging.
• Reorganized and updated the contributing section to provide clear guidance on how to contribute to the project and the adopted code of conduct.