updated README to explain book and website build better
updated README to explain book and website build better

Robinlovelace/spatial-microsim-book,richardellison/spatial-microsim-book,majazaloznik/spatial-microsim-book,jojo-/spatial-microsim-book,majazaloznik/spatial-microsim-book,Robinlovelace/spatial-microsim-book,emunozh/spatial-microsim-book,modumont/spatial-microsim-book,emunozh/spatial-microsim-book,modumont/spatial-microsim-book,jojo-/spatial-microsim-book,Robinlovelace/spatial-microsim-book,richardellison/spatial-microsim-book,modumont/spatial-microsim-book