TOSQUASH: add readme.txt in redirected opamroot

src/client/opamClient.ml

@@ -1143,7 +1143,7 @@ let init
     shell =
   log "INIT %a"
     (slog @@ OpamStd.Option.to_string OpamRepositoryBackend.to_string) repo;
-  let root =
+  let root, redirected =
     let root = OpamStateConfig.(!r.root_dir) in
     let has_space s = OpamStd.String.contains_char s ' ' in
     if Sys.win32 &&
@@ -1171,8 +1171,8 @@ let init
        let new_root = OpamFilename.Dir.of_string new_root_f in
        OpamFilename.write (OpamPath.redirected root) new_root_f;
        OpamStateConfig.update ~root_dir:new_root ();
-       new_root)
-    else root
+       new_root, true)
+    else root, false
   in
   let config_f = OpamPath.config root in
   let root_empty =
@@ -1193,6 +1193,12 @@ let init
           (OpamFilename.Dir.to_string root);
         if not (OpamConsole.confirm "Proceed?") then
           OpamStd.Sys.exit_because `Aborted);
+      (* Add the readme file in C:\opamroot as redirected *)
+      (if Sys.win32 && redirected then
+         OpamFilename.write
+           OpamFilename.Op.(root // "readme.txt")
+           (Printf.sprintf "Opam root redirected from %s"
+              (OpamFilename.Dir.to_string OpamStateConfig.(!r.root_dir)));
       try
         (* Create the content of ~/.opam/config *)
         let repos = match repo with