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/wrppm.c b/wrppm.c
index d3a613c..a930c81 100644
--- a/wrppm.c
+++ b/wrppm.c
@@ -6,7 +6,8 @@
  * Modified 2009 by Guido Vollbeding.
  * 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 write output images in PPM/PGM format.
  * The extended 2-byte-per-sample raw PPM/PGM formats are supported.