Request to add note to README.md about NSIS for Windows

[kevkha]

Without NSIS installed on Windows the gulp pack:win32 would fail with

[00:21:19] all done!
[00:21:19] Finished 'build:win32' after 6.87 s
[00:21:19] Starting 'pack:win32'...
'makensis' is not recognized as an internal or external command,
operable program or batch file.
[00:21:20] Finished 'pack:win32' after 200 ms

On Windows, download the nullsoft scriptable installer (http://nsis.sourceforge.net/Download) You need to include NSIS in your PATH to find makensis, set your global environment variable or you can set a session variable using:

set PATH=%PATH%;C:\Program Files (x86)\NSIS