Article 6 : Add PowerShell command to retrieve the installed Squirrel APP on a computer

This PowerShell command return the installed Squirrel App on a computer :
```powershell
Get-ChildItem -Path $env:localAppData -Recurse -Filter "Update.exe" | Select-Object {$_.Directory.Name}
```

We can integrate this information in the article about Squirrel.Windows.