Add note on temperature and milling flour (#132)

This just adds a small note on temperature when milling flour. The
provided source is an interesting read.

book/basics/how-sourdough-works.tex

@@ -38,7 +38,10 @@ activity so that it can thrive in its new environment.
 Of course, a ground flour can no longer sprout. But the enzymes that
 trigger this process are still present. That's why it's important not to
 mill grains at too high a temperature, as doing so could damage some of
-these enzymes.
+these enzymes.\footnote{In a recent study tests have shown that milling
+flour at home with a small mill had no significant negative impact on the resulting bread
+quality compared to milled flour from temperature-regulating large-scale mills.
+\ref{milling+commercial+home+mill+comparison}}
 
 Normally, the grain seed shields the germ against pathogens. However, as the
 grain is ground into flour, the contents of the seed are exposed. This is ideal