Add precisions to Windows README's install steps

Although the documentation to install Drush on Windows is complete (see http://drupal.org/node/594744 ), README doens't say that you need to install a few binaries in order to get Drush working properly.
This leads to several issues:
#776724: An error occured at drush_pm_download
#483110: Update fails silently on Windows (GNU Tar on Windows)
#330023: Install on Windows system

So here's a small patch in order to clarify the install steps.

I'm not sure regarding the step 2 of documentation saying you shoud use bsdtar for tar, so I let this out.