Add contributing guidelines

[v8p1197]

In order to guide users in adding new features or solving bugs, we should add a CONTRIBUTING.md file, in which we report contributing guidelines.
In particular, we should add instructions/indications about:

• Branch names/prefixes
• Pre-commit configuration
• Commit messages templates
• (Optional) PR/issue templates or, in general, indications on what to write in the PR/issue
• (Optional) Local Jenkins report to ensure building passes

The ros2_control one looks like a good example.