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/jpegtran.c b/jpegtran.c
index f978cef..f5ebd7b 100644
--- a/jpegtran.c
+++ b/jpegtran.c
@@ -5,7 +5,8 @@
  * Copyright (C) 1995-2010, Thomas G. Lane, Guido Vollbeding.
  * libjpeg-turbo Modifications:
  * Copyright (C) 2010, 2014, 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 a command-line user interface for JPEG transcoding.
  * It is very similar to cjpeg.c, and partly to djpeg.c, but provides