Not all plugins have a dedicated CONTRIBUTING.md

[astrojuanlu]

Description

I was confused when I saw that kedro-docker has a CONTRIBUTING.md https://github.com/kedro-org/kedro-plugins/blob/main/kedro-docker/CONTRIBUTING.md

but kedro-datasets doesn't.

On the other hand, the top-level CONTRIBUTING.md does not explain how to run the tests, but the kedro-docker specific one does https://github.com/kedro-org/kedro-plugins/blob/main/kedro-docker/CONTRIBUTING.md#ci--cd-and-running-checks-locally

Context

I was trying to tell a contributor (#355) how to run the tests only for one dataset.

[astrojuanlu]

Also, maybe instead of make test, we could tell contributors to run pytest? It's not only more explicit, it's actually faster to type...