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/djpeg.c b/djpeg.c
index d41f94f..8b516a6 100644
--- a/djpeg.c
+++ b/djpeg.c
@@ -6,7 +6,8 @@
  * libjpeg-turbo Modifications:
  * Copyright (C) 2010-2011, 2013-2015, D. R. Commander.
  * Copyright (C) 2015, Google, Inc.
- * 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 a command-line user interface for the JPEG decompressor.
  * It should work on any system with Unix- or MS-DOS-style command lines.