Adds a powershell install script for windows users that are at least
running powershell 5. It is almost a direct port of install.sh.
This would be used to automatically download the script much like
curl | bash, but with:
`Invoke-WebRequest https://urltoscript.com | Select-Object
-ExpandProperty Content | Out-File $env:TMP/install_rustlings.ps1;
Unblock-File $env:TMP/install_rustlings.ps1; Invoke-Expression
$env:TMP/install_rustlings.ps1`
George Doenlen