mention abi break in changelog

debian/changelog

@@ -8,6 +8,8 @@ apt (0.7.26~exp5) UNRELEASED; urgency=low
   * apt-pkg/contrib/fileutl.{h,cc}:
     - Add support for transparent reading of gzipped files.
     - Link against zlib (in apt-pkg/makefile) and add zlib build dependency.
+    - [ABI BREAK] This introduces a new protected member of FileFD and changes
+      the behaviour of FileFd for reading gzipped files.
   * apt-pkg/deb/debindexfile.cc:
     - If we do not find uncompressed package/source/translation indexes, look
       for gzip compressed ones.