Document howto generate completions

[andig]

https://github.com/spf13/cobra/blob/master/zsh_completions.md mentions its supported by root command. It would be great to document how to property create complextions for bash and zsh both, potentially switching on detecting the current shell.

[github-actions]

This issue is being marked as stale due to a long period of inactivity

[marckhouzam]

@andig The documentation for shell completion has extensively changed recently. An example of generating the different completion scripts can be found here:
https://github.com/spf13/cobra/blame/884edc58ad08083e6c9a505041695aa2c3ca2d7a/shell_completions.md#L56

Does the current documentation address your concerns?

[andig]

Does the current documentation address your concerns?

Just perfect, big thank you!