Refactor README for better readability

[Ved2009]

Updated README to improve clarity and formatting.

[Ved2009]

This PR cleans up the README for readability without changing any content or voice:

• Completed the "Contributing" section, which was cut off in the original (bug/issue reporting, PR guidance, connector pattern pointer)
• Added a table of contents for easier navigation given the doc's length
• Minor consistency fixes ("the App" → "the app", "API-keys" → "API keys" to match style used elsewhere)
• Reformatted the Tests/Desktop bundles section into a bullet list instead of one dense sentence

Happy to adjust scope or split into smaller PRs if preferred — let me know.