extend README

README.md

@@ -58,3 +58,11 @@ If you would like to know more about Iris, we recommend to take a look at:
   Ralf Jung, Robbert Krebbers, Jacques-Henri Jourdan, Aleš Bizjak, Lars
   Birkedal, Derek Dreyer.
   A detailed description of the Iris logic and its model
+
+## Generating the exercises
+
+If you want to contribute to the tutorial, note that the files in `exercises/`
+are generated from the corresponding files in `solutions/`. Run `make exercises`
+to re-generate those files. This requires `gawk` to be installed (which should
+usually be available on Linux but might have to be installed separately on
+macOS).