Document problems with Chef provisioner in README

As Packer currently has no support for WinRM and all communication
with the VM is done over SSH in Cygwin, some Packer features fail to
work properly including Chef. While Chef works for many packages, it
doesn't work with those that make assumptions about available programs
in PATH such as msiexec.exe.

This patch addresses the issue by documenting the problem in README.md
with hopes that future developers will spend less time trying to solve
Chef-related issues.
Grammar