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/jinclude.h b/jinclude.h
index 4dced6e..d461a1a 100644
--- a/jinclude.h
+++ b/jinclude.h
@@ -5,7 +5,8 @@
  * Copyright (C) 1991-1994, Thomas G. Lane.
  * 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 exists to provide a single place to fix any problems with
  * including the wrong system include files.  (Common problems are taken