Update README with usage guidance

[gitagentic[bot]]

This pull request addresses Issue #3 by updating the README.md file with more detailed usage guidance for Gitagentic.

Changes made:

• Added a new section "How Gitagentic Works" explaining how the bot handles issues, pull requests, and review comments.
• Included information about the configuration environment variables and their purposes.
• Explained the usage of labels for triggering the bot when WITH_LABEL is set to "yes".

These changes provide a clearer understanding of how to use and configure Gitagentic, addressing the requirements outlined in the issue.