README.md: instructions for compilation and testing

Some instructions should be inluded in the README.md to make clear how to get started with this. At this point content is limited to the generated images, which is useful but doesn't help a user get started.

I assume mkdir build && cmake -Bbuild -S. && cmake --build build.

Also needs prerequisities and dependency minimum versions (dd4hep minimum must be included in CMakeLists.txt too).