Document installing gawk on macOS

README.md

@@ -64,8 +64,8 @@ If you would like to know more about Iris, we recommend to take a look at:
 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).
+usually be available on Linux, and on macOS can be installed with
+`brew install gawk`).
 
 The syntax for the solution files is as follows:
 ```