Add note about error in Windows XP

[alexbaumgertner]

Error while adding Vagrant box in windows xp:

The box failed to unpackage properly. Please verify that the box
file you're trying to add is not corrupted and try again. The
output from attempting to unpackage (if any):

FIX: hashicorp/vagrant#3869 (comment)

@BakerCo: solution was to download the basic_bsdtar.exe from here http://sourceforge.net/projects/mingw/files/MinGW/Extension/bsdtar/ and rename it to bsdtar.exe and place it in C:\HashiCorp\Vagrant\embedded\gnuwin32\bin\