Fix discard instructions #409 (#410)

This changes the text slightly to make it clearer what to best do with
the discard that's piling up when making the starter initially.

book/sourdough-starter/sourdough-starter.tex

@@ -234,11 +234,22 @@ the starter when the starter itself dried out a lot.
 You will
 still have some mixture left from your first day. As this contains
 possibly dangerous pathogens that have been activated make sure you discard
-this mixture. Once your sourdough starter is mature you never need to
-discard it. It's long-fermented flour that is an excellent addon
+this mixture. A rule of thumb is to begin keeping the discard,
+the moment you made your first successful bread. At that point
+your discard is long-fermented flour that is an excellent addon
 used to make crackers, pancakes or delicious hearty sandwich
 bread\ldots I~also frequently dry it and use it as a rolling agent
-for pizzas that I~am making.
+for pizzas that I~am making.\footnote{Discarding starter when preparing
+a new batch can be frustrating. With experience, bread-making
+becomes more efficient, and excess discard is rarely produced. It is
+possible to prepare just the right amount of starter
+needed for bread dough. In fact, a fully depleted starter can even be revived
+using a small portion of bread dough. Any leftover discard, rich in spores,
+can also serve as a backup to create a new sourdough starter. Simply mix the
+discard with a little flour and water, and it will spring back to life. That is a
+great option if the starter was accidentally depleted. A practical approach
+is to store all discard in a single jar in the fridge, adding new discard on
+top as needed and using it whenever required.}
 
 You should hopefully again see some bubbles, the starter increasing
 in size and/or the starter changing its smell. Some people give