Add CONTRIBUTING.md

[trsonic]

Adds a contributing guide covering:

• CLA requirements and license terms
• Coding style: Google C++ style via the root clang-format config, with a note that the bundled obr library carries its own .clang-format
• Build, smoke test, and unit test steps to validate changes
• Pull request conventions (commit hygiene, rebase-and-merge policy)

[remi-audfray]

LGTM as well