Add Powershell Gallery to alternative source

add [powershell gallery](https://www.powershellgallery.com/) to the list of [alternative source](https://github.com/chocolatey/choco/wiki/CommandsInstall#alternative-sources)

everything in powershell gallery can be installed with [powershellget](https://github.com/PowerShell/PowerShellGet), which is built-in into powershell v5

Start with the CommandsInstall wiki page linked in the issue, specifically its alternative sources section. Add PowerShell Gallery as an alternative source and retain the PowerShellGet reference and link. Done means the page accurately explains that PowerShell Gallery content can be installed through PowerShellGet.

Written by the indexing model from the issue text.