README: document newgui2 build situation

README.md

@@ -6,3 +6,16 @@ A lot of documentation (wiki) and user forums were previously hosted at http://m
 see [issue #90](https://github.com/ygrek/mldonkey/issues/90) for the details and links to the data dumps (help needed to restore them to the usable form).
 
 [GitHub Discussions](https://github.com/ygrek/mldonkey/discussions) serves as a replacement for the forum for the time being.
+
+Known issues
+============
+
+GTK2 GUI (./configure --enable-gui=newgui2) builds only in specific configuration when lablgtk2 packages are installed in $(ocamlc -where)/lablgtk2
+This happens to be true on Debian when building with packaged (system) ocaml :
+
+  sudo apt install ocaml camlp4 libnum-ocaml-dev liblablgtk2-gnome-ocaml-dev
+
+It doesn't work with opam - this is the goal for the next release.
+
+`./configure --enable-batch --enable-gui=newgui2` (ie download and build lablgtk2 locally) only works with old OCaml versions (unsafe-string)
+because it is using very old lablgtk2.  Upgrading to newer lablgtk2 is the goal for the next release