Now that we have [PowerShell Core](https://github.com/powershell/powershell) the cross-plat version of PowerShell... it'd be really cool to have a badge that shows the OS's that your module works on. On the PowerShell Gallery, users can do this by using the tags `Windows` `MacOS` `Linux`. In this PR, I use the PowerShell Gallery API to grab the tags from that package and display `windows` `macos` `linux` if they exist. The result is:  which aligns with Conda and Cocoapods