Add markdown file explaining support for PowerShell

powershell_completions.md

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+# Generating PowerShell Completions For Your Own cobra.Command
+
+Cobra can generate PowerShell completion scripts. Users need PowerShell version 5.0 or above, which comes with Windows 10 and can be downloaded separately for Windows 7 or 8.1. They can then write the completions to a file and source this file from their PowerShell profile, which is referenced by the `$Profile` environment variable. See `Get-Help about_Profiles` for more info about PowerShell profiles.
+
+# What's supported
+
+- Completion for subcommands using their `.Short` description
+- Completion for non-hidden flags using their `.Name` and `.Shorthand`
+
+# What's not yet supported
+
+- Command aliases
+- Required, filename or custom flags (they will work like normal flags)
+- Custom completion scripts