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/jdcolor.c b/jdcolor.c
index 38db90f..ac96b23 100644
--- a/jdcolor.c
+++ b/jdcolor.c
@@ -8,7 +8,8 @@
  * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
  * Copyright (C) 2009, 2011-2012, 2014-2015, D. R. Commander.
  * Copyright (C) 2013, Linaro Limited.
- * 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 output colorspace conversion routines.
  */