INSTALL: adding infos about `dot` for "gegl:introspect" dependency.

Especially now that this is not a blocking build dependency on GEGL, we
want to have the information somewhere so that packagers (and other
people building GIMP themselves) know this is at least an optional
runtime dependency.

INSTALL.in

@@ -57,6 +57,10 @@ header files installed.
        the option -Dworkshop=true in order to be able to select the
        experimental Paint Select tool in the Playground (operation
        "gegl:paint-select" is needed).
+     - The "Show Image Graph" item in the "Debug" menu (hidden by
+       default on stable release) requires the GEGL operation
+       "gegl:introspect" which is always built but deactivated unless
+       the `dot` tool from graphviz is available (runtime dependency).
 
   4. You need to have installed GTK+ version @GTK_REQUIRED_VERSION@ or newer.
      GIMP also needs a recent version of GLib (>= @GLIB_REQUIRED_VERSION@), GDK-Pixbuf