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/ChangeLog.txt b/ChangeLog.txt
index 5ae9431..9ebbfa9 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -441,7 +441,7 @@
 incremented by 1 to reflect this.  You can disable this feature with a
 configure/CMake switch in order to retain strict API/ABI compatibility with the
 libjpeg v6b or v7 API/ABI (or with previous versions of libjpeg-turbo.)  See
-README-turbo.txt for more details.
+README.md for more details.
 
 [13] Added ARMv7s architecture to libjpeg.a and libturbojpeg.a in the official
 libjpeg-turbo binary package for OS X, so that those libraries can be used to
@@ -677,7 +677,7 @@
 ==================
 
 [1] Added emulation of the libjpeg v7 and v8 APIs and ABIs.  See
-README-turbo.txt for more details.  This feature was sponsored by CamTrace SAS.
+README.md for more details.  This feature was sponsored by CamTrace SAS.
 
 [2] Created a new CMake-based build system for the Visual C++ and MinGW builds.