Open source Key files

[annetill]

Open-source organizations required some key files, in priority order:

1. Before adding repositories with lines of code:

• A PR template (example https://github.com/powsybl/.github/blob/main/PULL_REQUEST_TEMPLATE.md)
• A code of conduct
• A simple contributing file mentioning code of conduct, English convention, licence, contributing process with DCO, branch workflow + review + merge with squash of commits and a temporary mention to no contribution external from the organization for the moment as we are waiting for a clear governance.

2. Can be done just before the first release:

• A maintainers file
• A security file

3. Can be done later:
   An organization README file (in a "profile" repository)