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/jcapimin.c b/jcapimin.c
index 3b005d3..3e68ce3 100644
--- a/jcapimin.c
+++ b/jcapimin.c
@@ -6,7 +6,8 @@
  * Modified 2003-2010 by Guido Vollbeding.
  * It was modified by The libjpeg-turbo Project to include only code relevant
  * to libjpeg-turbo.
- * 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 application interface code for the compression half
  * of the JPEG library.  These are the "minimum" API routines that may be