Dedicated Luigi npm packages README files

[hardl]

... for core and client.
For Luigi client there is already a README file: https://github.com/kyma-project/luigi/tree/master/client, which is then used in the luigi-client package.
For Luigi core, we need to add a basic README file here: https://github.com/kyma-project/luigi/tree/master/core/public with some general infos about luigi and links to github.