Improve set up instructions in the README file

The goal is to add some details to the README file to make it clear and facilitate the set up process.

Setup Maven and NPM repositories

• Which permissions should we give to the GitHub token? At least read and write? We can probably try this to understand the problem. 

• How to use docker to start a NOS image?
• Add a new section in the document before https://github.com/nuxeo/nos#docker-nos-start-script to add the docker registry to fetch the NOS container images (since the repository is private, we can have that info in the repository)
• Example:
  > docker login docker-nos-private.packages.nuxeo.com