change the README.md file to include more info on FFTW

Author: jayponder

README.md

@@ -14,4 +14,6 @@ Tinker is the major release of the Ponder Lab tool set for molecular mechanics a
 
 <H2><B>Building from Source Code</B></H2>
 
-Tinker is provided as a complete source distribution. After unpacking the release, you can build a set of Tinker executables on almost any machine with GNU gfortran or another modern Fortran compiler. Makefiles, a cmake configure script, as well as standalone scripts to compile, build object libraries, and link executables on a wide variety of machine-CPU-operating system combinations are provided. To build using the Tinker Makefile, just copy /make/Makefile into the /source area of the distribution. Small changes are needed near the top of the Makefile to set directory names and activate the appropriate operating system and compiler. Then issue the "make" command while in the /source directory. Note the FFTW libraries must be available before building Tinker. Pre-build FFTW libraries for some operating system and compiler combinations are provided with the source distribution.
+Tinker is provided as a complete source distribution. After unpacking the release, you can build a set of Tinker executables on almost any machine with GNU gfortran or another modern Fortran compiler. Makefiles, a cmake configure script, as well as standalone scripts to compile, build object libraries, and link executables on a wide variety of machine-CPU-operating system combinations are provided. To build using the Tinker Makefile, just copy /make/Makefile into the /source area of the distribution. Small changes are needed near the top of the Makefile to set directory names and activate the appropriate operating system and compiler. Then issue the "make" command while in the /source directory.
+
+Note the FFTW libraries must be available prior to building Tinker. Pre-built FFTW libraries for some operating system and compiler combinations are provided under the /lib sibdirectory of the source distribution. Or you can build the libraries following the instructions in the file /fftw/0README.