Add Python dependency to README.windows

README.windows.md

@@ -48,6 +48,8 @@ or [64-bit](http:https://sourceforge.net/projects/mingwbuilds/files/host-windows/relea
 3. Add the line "C:/MinGW /mingw" to C:/MinGW/msys/1.0/etc/fstab (create the file if it doesn't exist)
 4. You will need to replace C:/MinGW/msys/1.0/bin/make.exe with C:/MinGW/msys/1.0/bin/make-old.exe or with a copy of make.exe extracted from [mingw-msys](http:https://sourceforge.net/projects/mingw/files/MSYS/Base/make/make-3.81-3/) (e.g. make-3.81-3-msys-1.0.13-bin.tar.lzma)
 
+Before proceeding, verify that python.exe is available in the MSYS PATH. If Python is not installed on your computer, [download Python 2.7](http:https://www.python.org/download/releases/2.7.5/) and install with default options (Python is required for LLVM build).
+
 These sections assume you are familiar with building code. If you are not, you should stop reading now and go the the section on binaries. Regardless of which set of steps you followed above, you are now ready to compile julia. Open a unix shell by launching C:/MinGW/msys/1.0/msys.bat (or your favorite shortcut to that file). 
 
 Run the following commands in your build directory ($HOME at C:/MinGW/msys/1.0/home/your_name is fine)