make update_whatsnew script executable

and note where the script is in whatsnew/pr/README

docs/source/whatsnew/pr/README.md

@@ -16,3 +16,5 b' have a filename starting with `incompat-`, such as'
 `incompat-switching-to-perl.rst`.  Our "What's new" files always have two
 sections,  and this prefix scheme will make sure that the backwards incompatible
 changes get routed to their proper section.
+
+To merge these files into :file:`whatsnew/development.rst`, run the script :file:`tools/update_whatsnew.py`.

tools/update_whatsnew.py

@@ -1,3 +1,4 b''
+#!/usr/bin/env python
 """Update the What's New doc (development version)
 
 This collects the snippets from whatsnew/pr/, moves their content into