(Doc) Update ReadMe to give a better idea of what each project is about.

[kdesnos]

As the number of projects in Preesm-Apps grows, it becomes more and more difficult to know what each project is about.

The root ReadMe file should give a very short description of what each project is. In addition to the "purpose" of the application it represents, it could indicate whether it is a project for Preesm, Spider, or both.

Internal, Readmes of individual project should also be updated as some of them are simple copies of the "default" sobel Readme file.