Published npm packages don't feature a README.md file

Published npm packages don't contain a README.md file. Consequently, the package pages on npmjs.com are not really nice.
For instance, at the time of writing, the page for https://www.npmjs.com/package/@xwiki/platform-document-api looks like this:

This is not user-friendly, and we need to think of a way to have a README file with at least a link to our documentation.