[doc] Add summary for the release notes

Author: Pierre-Sassoulas

doc/whatsnew/2/2.15/index.rst

@@ -11,7 +11,10 @@
 Summary -- Release highlights
 =============================
 
-* We improved ``pylint``'s handling of namespace packages. More packages should be
-  linted without resorting to using the ``-recursive=y`` option.
+In pylint 2.15.0, we added a new check ``missing-timeout`` to warn of
+default timeout values that could cause a program to be hanging indefinitely.
+
+>e improved ``pylint``'s handling of namespace packages. More packages should
+be linted without resorting to using the ``-recursive=y`` option.
 
 .. towncrier release notes start

doc/whatsnew/fragments/6974.internal

@@ -0,0 +1,5 @@
+We changed the way we handle the changelog internally by using towncrier.
+If you're a contributor you won't have to fix merge conflicts in the
+changelog anymore.
+
+Closes #6974