Revise contributing section in README.md

[ChernegaSergiy]

This pull request updates the contributing guidelines in the README.md to make the onboarding process more welcoming and clarify expectations for new contributors.

Major Changes

• Enhanced Welcome Message: Updated the introductory text in the contributing section to be more encouraging and appreciative of community efforts.
• Quality & Testing Guidelines: Added an explicit requirement for contributors to update tests as appropriate and adhere to the project's existing coding style before submitting a pull request.

Additional Improvements

• Formatting Polish: Standardized the markdown syntax for the numbered list of contribution steps to improve readability and consistency within the document.