Help: Add 4.2 release note for CPack AppImage generator

This was missed in commit 1a6dbcc9ea (CPack: Add AppImage generator,
2025-07-30, v4.2.0-rc1~229^2).

Issue: #27104

Help/release/4.2.rst

@@ -172,6 +172,8 @@ Generator Expressions
 CPack
 -----
 
+* The :cpack_gen:`CPack AppImage Generator` was added.
+
 * The :variable:`CPACK_PACKAGE_CHECKSUM` variable now supports multiple values.
 
 * The :cpack_gen:`CPack NSIS Generator` gained a