Document problems with Chef provisioner in README

[vmrob]

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.

This addresses the issue raised here: #101

[kensykora]

Merging into develop. I'll fix the grammar error as well. Thanks for the PR!

[kensykora]

Merged 9824a4a

[stonith]

@vmrob I've gotten around the broken chef behaviour by executing chef via winrs via ssh. Dirty hack but seems to work for me.

          "type": "shell",
            "remote_path": "/cygdrive/c/Windows/Temp/script.bat",
            "execute_command": "winrs -u:vagrant -p:vagrant c:/windows/temp/script.bat",
            "scripts": [
                "./scripts/run-chef.bat",
                "./scripts/run-winupdates.bat"
            ]
        },

[kensykora]

@stonith I've needed to perform similar behaviors on Puppet as well.