Add more pH Meter details (#367)

See issue #332 for details

book/wheat-sourdough/wheat-sourdough.tex

@@ -913,7 +913,12 @@ \section{Bulk fermentation}%
 that work for your setup.
 
 Another limitation is the price. You will need to purchase
-a high-tech pH meter, ideally, a meter featuring a spearhead.
+a high-tech pH meter, ideally, a meter featuring a spearhead
+\footnote{Not every pH meter is suitable for measuring dough.
+Please refer to the manual to make sure it is certified for
+measuring the pH of liquid and semi-solid media. To receive
+accurate pH readings further ensure that your pH meter
+is properly calibrated.}.
 This way you can directly poke the meter deep into the dough.
 At the same time, automated temperature adjustments are a
 feature to look out for. Depending on the temperature,