Add information how to deal with incorporation external code (license, credits)

Describe the enhancement or feature you'd like
The devguide does not contain information what a triager and core dev has to do when adding files or non-trivial portions of code from an external project with non-core dev contributors. There are a couple of things we need to check or do:

• Is the license compatible with PSFL?
• Include proper provenance and license information in file header or comment before the code
• Add license and origin to Docs/license.rst
• Give credits to original authors
• Properly document the provenance and license in commit message

Describe alternatives you've considered
We should ask VanL for guidance, too.

Additional context
See https://github.com/python/cpython/issues/95649 for a case that is missing proper license and provenance information.

まず devguide ドキュメントの構成と、関連する CPython issue 95649 を確認し、その後 VanL に想定されているガイダンスを尋ねます。手順が完全で実行可能なものになるよう、ライセンス互換性、出所、クレジット、Docs/license.rst、およびコミットメッセージに関する要件を文書化します。