mention deps in generate_classes docs

.docs/md/generate_classes.md

@@ -10,7 +10,11 @@
 
 MODFLOW 6 input continues to evolve as new models, packages, and options are developed, updated, and supported. All MODFLOW 6 input is described by DFN (definition) files, which are simple text files that describe the blocks and keywords in each input file. These definition files are used to build the input and output guide for MODFLOW 6. These definition files are also used to automatically generate FloPy classes for creating, reading and writing MODFLOW 6 models, packages, and options.  FloPy and MODFLOW 6 are kept in sync by these DFN (definition) files, and therefore, it may be necessary for a user to update FloPy using a custom set of definition files, or a set of definition files from a previous release.
 
-The FloPy classes for MODFLOW 6 are largely generated by a utility which converts DFN files in a modflow6 repository on GitHub or on the local machine into Python source files in your local FloPy install. For instance (output much abbreviated):
+The FloPy classes for MODFLOW 6 are largely generated by a utility which converts DFN files in a modflow6 repository on GitHub or on the local machine into Python source files in your local FloPy install.
+
+**Note**: to use this functionality, the `codegen` optional dependency group must be installed.
+
+For instance (output much abbreviated):
 
 ```bash
 $ python -m flopy.mf6.utils.generate_classes