docs: update LICENSE and CONTRIBUTING to clarify contribution licenses

[hyperfinitism]

Summary

This PR updates the LICENSE and CONTRIBUTING.md files to clarify the licensing terms and copyright ownership for contributions.

Changes

LICENSE

• Added "and The py-snp-utils Contributors" to the copyright notice to explicitly acknowledge that contributors retain copyright over their contributions.

CONTRIBUTING.md

• Renamed the section "Contribution Agreement" to "License of Contributions".
• Added explicit statement that contributors retain copyright to their contributions.

Motivation

These changes aim to:

1. Clarify copyright ownership: Make it explicit that the codebase's copyright belongs not only to Acompany Co., Ltd. (the initial developer) but also to each contributor.
2. Clarify rights retention: Ensure contributors understand they retain full copyright to their contributions while licensing them under MIT.