Add current preparation PR itself to notes

Author: lagru

docs/release_notes/v0.4.0.md

@@ -46,6 +46,7 @@ Find a more detailed list of pull requests contributing to this release below.
 ## Maintenance
 
 - Improve walking of the source directory ([#68](https://github.com/scientific-python/docstub/pull/68)).
+- Prepare release of version 0.4.0 ([#72](https://github.com/scientific-python/docstub/pull/72)).
 
 ## Contributors