Update PowerShell File Extensions

The three core PowerShell language extensions are .psd1, .ps1 and .psm1 -- plus two xml file extensions: .ps1xml and .clixml which are for formatting rules and serialization.
.psm1 modules files use exactly the same syntax as scripts, but are imported rather than executed.
.psd1 are metadata files which use a subset of the same syntax (they can be highlighted using the same highlighter, it's just some commands, variables, and types aren't allowed in data files)

I don't know why there's not a page on MSDN listing all the file types like this: 
https://en.wikipedia.org/wiki/Windows_PowerShell#File_extensions

But there are pages about each one, if you look carefully:

- ps1xml
  - http://msdn.microsoft.com/en-us/library/dd878306 
  - http://msdn.microsoft.com/en-us/library/hh847881
- psm1 
  - http://msdn.microsoft.com/en-us/library/dd901839
  - http://msdn.microsoft.com/en-us/library/hh847841
- psd1 
  - http://msdn.microsoft.com/en-us/library/dd878297
  - http://msdn.microsoft.com/en-us/library/hh849919