Improve README

[skorsky]

Some topics which should be covered in https://github.com/HEADS-project/training/blob/master/README.md.

1. Describe the settings of the Eclipse projects: What kind of project do I need? What are the natures of the projects.
2. What is roughly the project content (which folders are generated, where are the asset to trigger actions)?
3. Which actions do I have to do in HEADS IDE? Which actions do I have to do in a shell?

In addition https://github.com/HEADS-project/training/blob/master/3.Wrapping_ThingML_into_Kevoree/README.md should mention that instructions to build and run are in the thingml files.