Update README information for the libmbfl library

The libmbfl library is bundled with PHP and has its own repository for
development and bug fixes. To avoid confusion and faster development the
README has been updated to include the information of the original library and
to use the bundled library as a fork of the upstream repository instead.

Author: petk

ext/mbstring/libmbfl/README

@@ -1,8 +1,13 @@
-This is Libmbfl, a streamable multibyte character code filter and
-converter library.
+This is libmbfl, a streamable multibyte character code filter and converter
+library.
 
-See LICENSE and DISCLAIMER for licensing information.
+The original version of libmbfl is developed and distributed at
+https://github.com/moriyoshi/libmbfl under the LGPL 2.1 license.
+
+The libmbfl library is bundled with PHP as a fork of the original repository and
+is not in sync with the upstream at https://github.com/moriyoshi/libmbfl. As such,
+the libmbfl directory is directly modified in the php-src repository.
 
-See the file INSTALL for building and installation instructions.
+See LICENSE and DISCLAIMER for licensing information.
 
-# $Id$
+See INSTALL for building and installation instructions.