Document docker executor and npm min version

🌳 Context

During a recent debugging session with customer, we realized that:

• The importer uses $ npm commands or options that are available only from npm 7. Any older $ npm will not work.
• The image: option will only work if the pipeline is executed by a Docker executor. For example, if a shell execution is used, that image: option will have no impact.

🤔 What does this MR do?

• Update the README.md file with the answer for the above points.