README: add a note about broken hg-git compatibility mode

README.asciidoc

@@ -71,6 +71,20 @@ the same commits:
 % git config --global remote-hg.hg-git-compat true
 --------------------------------------
 
+****
+mnauw's note; The above is not quite the case, it only ever has been (somewhat) if
+an undocumented debug mode was enabled in `hg-git` (`debugextrainmessage` setting),
+and in recent versions the latter is no longer considered.  In fact, `hg-git` creates
+git commits with additional hg metadata stored in so-called "extra commit headers".
+The latter might be seen by `git log --format=raw` or `git cat-file -p <commitid>`,
+but are otherwise mostly used internally by the git suite (for signatures).  However,
+there is limited to no support for those in git "porcelain or plumbing" commands.
+In particular, `git fast-export` and `git fast-import` do not consider these,
+so a `gitremote-helpers` tool is then also out of luck.  Incidentally, it also
+follows that a `git fast-export | git fast-import` "clone" approach would also
+lose such extra metadata, and likewise so for e.g. `git filter-repo`.
+****
+
 == Notes ==
 
 Remember to run `git gc --aggressive` after cloning a repository, especially if