Fixed doc/requirements.md to include PowerShell and PowerShell Community Extensions under Windows

1) Fixed doc/requirements.md to include installing PowerShell and the PowerShell
   Community Extensions (PSCX) under Windows. PowerShell is used for scripting
   under Windows, and PSCX supplies several necessary cmdlets including
   Write-Tar.
Merge pull request #773 from markleightonfisher/dev-mlf

Fixed doc/requirements.md to include PowerShell and PowerShell Community...