Replace README with README.md (third draft)

This pull request adds a `README.md` file, replacing old `README`. It is not intended to be merged but to continue a discussion about how YaST module `README.md` files should generally look like, which was started by [this pull request](https://github.com/yast/yast-nfs-server/pull/7).

The added `README.md` file is a draft with missing links and some content.