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/wrbmp.c b/wrbmp.c
index b7ecb49..06630d8 100644
--- a/wrbmp.c
+++ b/wrbmp.c
@@ -6,7 +6,8 @@
  * libjpeg-turbo Modifications:
  * Copyright (C) 2013, Linaro Limited.
  * Copyright (C) 2014, 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 routines to write output images in Microsoft "BMP"
  * format (MS Windows 3.x and OS/2 1.x flavors).