Some more README restructuration

This time I think I am clear on what the building & running process is
and what the purpose of each command is, and I tried to make it clear again.

This may be deemed too verbose:
- Maybe I went into too many basic details, I'll be happy to discuss
  what the right level of detail is;
- But I also think there are too many ways to do the same thing
  documented in the README. It might be worth settling on one recommended way,
  and documenting the rest in an appendix or something like that.