Explain directory layout in README.md

[dev-zero]

There was some confusion on the directories, a quick overview might be beneficial. Also, when running python solutions/smartsim_101.py this will pick up the simulation from src/cpp/... rather than from the solution (which may be intentional as one could first try to augment the simulation before looking at the driver).

[al-rigazzi]

Good suggestion. I think we can fix this by using absolute paths.