docs: clarify include paths

CONTRIBUTING.md

@@ -40,6 +40,11 @@ Recommended:
 
 - Use `doxygen` (or `make docs`) to generate documentation you can reference when developing.
 
+#### Note on include paths
+
+The current working directory of all entrypoints in the `sites` directory is anchored to `sites` itself.
+This ensures that references to components are not broken when the page changes locations. For example, `require '+getenv.php';` works in both `index.php` and `about/index.php`. However, as a result, files included from such endpoints (even from the `php/` subtree) may have their parent folders shadowed when trying to use `include` themselves. Files in the same folder and in subfolders can be included, however. How PHP's `include` fallback mechanism implies this behavior is unclear.
+
 ### Running your own copy
 
 - point your webserver at the [`output`](output/) folder