Create a central contributing document

[jodh-intel]

We need to create a CONTRIBUTING.md document in this repository whose content can be applied to all the other Kata (code) repositories.

Once done, we can create a minimal CONTRIBUTING.md in each other repo that says something like:

For details on how to contribute to the Kata Containers project, please see the main contributing document.

References:

• Contributing proxy#14
• Contributing Guidelines proxy#15