Rename README, LICENSE, BUILDING text files

The IJG README file has been renamed to README.ijg, in order to avoid
confusion (many people were assuming that that was our project's README
file and weren't reading README-turbo.txt) and to lay the groundwork for
markdown versions of the libjpeg-turbo README and build instructions.
diff --git a/jdatadst.c b/jdatadst.c
index 7a40e4f..f078b7d 100644
--- a/jdatadst.c
+++ b/jdatadst.c
@@ -6,7 +6,8 @@
  * Modified 2009-2012 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
  * Copyright (C) 2013, D. R. Commander.
- * For conditions of distribution and use, see the accompanying README file.
+ * For conditions of distribution and use, see the accompanying README.ijg
+ * file.
  *
  * This file contains compression data destination routines for the case of
  * emitting JPEG data to memory or to a file (or any stdio stream).