Adding build requirements for windows to README.md

@brentlintner, Good news! Build for windows was fixed a long time ago - https://github.com/apache/incubator-ripple/commit/416a7fa5571061ff3e430d74cdb5fa8ab3fb7dad . Eventhough it was a bit tricky to build it due to jsdom / contextify visual studio requirement, which must be installed in a subtle way -  https://github.com/brianmcd/contextify/wiki/Windows-Installation-Guide . So, I do believe it's a good idea to update existing README.MD with the windows build requirements.