Update Readme to indicate git bash installation requirement

[TJM]

I believe that I figured this out by looking through the "issues" but it might be useful to update the readme to indicate that you need to check the NON-DEFAULT option Use Git and optional Unix tools from the Windows Command Prompt. It does indicate in the installation to not check that unless you understand the implications (not sure I do) of a couple typical dos utilities (find and sort) being overridden by the gnu utilities of the same name.