Update docs/developer-guide/index.md

Co-authored-by: David Glick <[email protected]>

docs/developer-guide/index.md

@@ -11,6 +11,12 @@ myst:
 
 This part of the documentation provides information for how to develop in Plone.
 
+```{note}
+This part of the documentation is under construction.
+You can find documentation for development in various locations through the [search feature](https://6.docs.plone.org/search.html?q=development).
+You can help consolidate all of development documentation here, even if it is to let us know what is missing, by [creating an issue](https://github.com/plone/documentation/issues/new?assignees=&labels=&projects=&template=new-issue-form.yml).
+```
+
 
 ```{toctree}
 :maxdepth: 2