Update Contributing.MD to describe how to use attribute registry when defining new semconv.

[jsuereth]

Today, CONTRIBUTING asks users to fill out yaml files for metrics and definitions, but doesn't describe the new attribute registry layout we're moving towards. We should update that with guidance on where to put attributes and how to reference them.

cc @AlexanderWert

[AlexanderWert]

Will write something for that

[joaopgrassi]

We don't specifically talk about the registry, but we say changes must be done in model. https://github.com/open-telemetry/semantic-conventions/blob/main/CONTRIBUTING.md#how-to-contribute. Do you think this is enough, or should we modify it still to say:

• Attributes should be added to registry
• Attributes must be referenced from the registry using ref.