Add a index.php showing a note about the "public" directory

This avoids that users get a 403 error (if DirectoryIndex is disabled) or a directory listing after updating to 3.2. This is what likely happened in https://github.com/postfixadmin/postfixadmin/issues/30 (see the comment from Apr 24, 2018)
8 years ago · 7f19cae57e
1 changed files with 9 additions and 0 deletions
--- a/index.php
+++ b/index.php
@ -0,0 +1,9 @@
+<html>
+<head>
+    <title>Postfix Admin</title>
+</head>
+<body>
+    <p>The Postfix Admin directory layout changed.</p>
+    <p>Please update your webserver config so that the DocumentRoot or Alias points to the directory "public".</p>
+</body>
+</html>