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: Idab5330d7257ccdacafec85640e8b559bc5b4b4b
diff --git a/CHANGELOG b/CHANGELOG
index f2a3ad2..f6efdc3 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,6 @@
+yyyy-mm-dd next version
+  This release is ABI compatible with the last release.
+
 2025-09-05 v3.13.1
   This release is ABI compatible with the last release.