rename project_setup.md

[jspaaks]

project_setup is not very clear to me, it seems to indicate something about how to set up a project, which at the time users learn about this file, they actually just did so they're not interested anymore. Of course it helps that we now point to this file from the posthook, but I feel the file name could be clearer as well.

I don't have any great alternatives, but here's a couple candidates:

1. after_project_generation.md
2. about_project_layout.md
3. project_layout_explained.md

Out of these, I think I prefer 3, unless others have better suggestions?

[jspaaks]

Also, we could consider prepending whatever name we choose with README, e.g. README.project_layout_explained.md such that the file is listed next to README.md and README.dev.md

[jspaaks]

Absent suggestions, made PR #294

[egpbos]

I think the file should be removed from the generated project altogether, see #310.