Add notebook interdependencies in README

[anastassiavybornova]

Make clear in README whether results from previous notebooks are needed to run a specific notebook (and if so, which)
e.g. nb 8 won't run if nb 7 wasn't run, I think? but not sure, would be good to specify

[mszell]

Thx, yeah, the later notebooks are messy in this regard, because stuff was added later-on (maybe due to paper reviews etc). So, this might require a larger rewrite/reordering.

[mszell]

Updated with: 246ba32
Optional notebooks are now named X_*. In the future, they could be integrated into previous notebooks, but that's some work. Leaving this open.