Added documentation for enabling unencrypted WinRM

Per GH#4752, unencrypted WinRM access must be configured on the guest
machine before Vagrant can communicate with the box. This is sometimes
enabled by default on public boxes, but needs to be documented for
those of us that are using custom made boxes.
Merge pull request #5025 from zmingee/improved-windows-guest-documentation

Added documentation for enabling unencrypted WinRM