Add organisation repository and `README.md`

I would like to see an information page (`README.md`) for the GitHub organization.

This can be realized by creating a repository with the name of the organization (`dotnet`) and creating a `README.md` in it.

This repository can then be used for organization specific things like more docs, ... (and possibly as an issue tracker for organization specific things - like this issue which doesn't really fit to dotnet/core).

### Example: GitHub - https://github.com/github/

/cc @terrajobst