Add initial documentation, directory structure

Organizing the repository a bit. It is nice to let people know what is
the purpose of this repository:

  - Update main README.md to be more helpful
  - Add docs/ sub-directory
  - Add tracing/ sub-directory
  - Add sceth/ sub-directory
  - Add 'trace-cmd' as a submodule under tracing/

Now that the basic structure is ready, the next commits should add
some code, to reflect the current status of this project.