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/rdrle.c b/rdrle.c
index 8df3ddb..bb14879 100644
--- a/rdrle.c
+++ b/rdrle.c
@@ -5,7 +5,8 @@
  * Copyright (C) 1991-1996, Thomas G. Lane.
  * It was modified by The libjpeg-turbo Project to include only code and
  * information 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 routines to read input images in Utah RLE format.
  * The Utah Raster Toolkit library is required (version 3.1 or later).