Add a placeholder for next version in CHANGELOG

This allows us to start describing changes in the next libaom release.

Note: The second word in this line must not start with "v", otherwise
build/cmake/version.pl will parse the second word as the current
version.

Change-Id: Iafc1317249e71e68b9f78617445b814a5dd5504e

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index dcaa449..88b57b6 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,6 @@
+yyyy-mm-dd next version
+  This release is ABI compatible with the last release.
+
 2026-10-06 v3.15.2
   This release is ABI compatible with the last release. See
   https://aomedia.googlesource.com/aom/+log/v3.15.1..v3.15.2 for all the