Add instructions for building from release tarball, no autoreconf required.

BUILDING.txt

@@ -68,12 +68,25 @@ The following procedure will build mozjpeg on Linux, FreeBSD, Cygwin, and
 Solaris/x86 systems (on Solaris, this generates a 32-bit library.  See below
 for 64-bit build instructions.)
 
+Simple Release tar.gz Source Build
+----------------------------------
+
+  cd {source_directory}
+  ./configure [additional configure flags]
+  make
+
+Non-Release Source Build (e.g. GitHub clone)
+--------------------------------------------
+
   cd {source_directory}
   autoreconf -fiv
   cd {build_directory}
   sh {source_directory}/configure [additional configure flags]
   make
 
+Results
+-------
+
 This will generate the following files under .libs/
 
   libjpeg.a