Update `README.md`

[Nielswps]

The README.md is currently outdated. It should be updated to use examples instead of samples and Reveaal is currently misspelled. I also think it would be advantageous to include a link to the Ecdar/ECDAR repository for downloading releases.

All in all, the following things should be updated/added:

• [x] Correct spelling mistakes
• [x] Add an information box pointing to ECDAR if the goal is usage and not development - [ ] Rewrite Sample Projects to use examples and maybe a short description of what is included, what they can be used for, etc.
• [x] Rewrite Engine Configuration to include how to actually configure the engines in the GUI
  • [x] Default engines are automatically picked up on the first startup
  • [x] How to add custom engines
• [x] Add a Contributing section to inform new developers about how they best contribute to the project

[Nielswps]

An in-depth description of the the exsamples will be covered by adding a reference to the paper that covers all the theoretical aspects currently available in the system, which is currently a WIP