Add doxygen to development instructions

CONTRIBUTING.md

@@ -9,6 +9,7 @@
 - `entr` to watch for file changes
 - `xdg-open` link handler to invoke browser
 - `libgd` (`php-gd`, `phpX.Y-gd`) for downsizing images
+- (Optional) `doxygen` to generate documentation
 
 ### Official repositories
 
@@ -37,6 +38,8 @@ Recommended:
 
 - Symlink the commit hooks provided in [`etc/hooks`](etc/hooks/) to `.git/hooks/<hook>` to run a full test cycle when committing to main.
 
+- Use `doxygen` (or `make docs`) to generate documentation you can reference when developing.
+
 ### Running your own copy
 
 - point your webserver at the [`output`](output/) folder