added shell completion instructions

Author: mikefarah

cmd/shell_completion.go

@@ -13,14 +13,29 @@ func createBashCompletionCmd(rootCmd *cobra.Command) *cobra.Command {
 	var completionCmd = &cobra.Command{
 		Use:   "shell-completion",
 		Short: "Generates shell completion scripts",
-		Long: `Example usage (for bash): to load completion run
-	
+		Long: `To load completion for:
+bash:
+	Run	
 	. <(yq bash-completion)
 	
-	To configure your bash shell to load completions for each session add to your bashrc
+	To configure your bash shell to load completions for each session add to
+	your bashrc
 	
 	# ~/.bashrc or ~/.profile
 	. <(yq bash-completion)
+
+zsh:
+	The generated completion script should be put somewhere in your $fpath named _yq
+
+powershell:
+	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.
+
+fish:
+	Save the output to a fish file and add it to your completions directory.
+
 	`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			switch shellVariant {