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/jcdctmgr.c b/jcdctmgr.c
index 4cac666..73f3ea4 100644
--- a/jcdctmgr.c
+++ b/jcdctmgr.c
@@ -7,7 +7,8 @@
  * Copyright (C) 1999-2006, MIYASAKA Masaru.
  * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
  * Copyright (C) 2011, 2014-2015 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 the forward-DCT management logic.
  * This code selects a particular DCT implementation to be used,