The plugin should echo helpful post-install instructions

[clmay]

Since the plugin requires additional commands to be run after installation in order to function (e.g. asdf direnv setup --shell bash --version latest), it would be really helpful if the install command printed guidance w.r.t. next steps.

The only reason I'm here on the GH repo is to figure out why asdf install direnv didn't give me a working direnv. Finding the answer via the README was easy enough, but seems like an unnecessary hoop for the user.

I'm not terribly familiar with how asdf plugins work, but I assume the asdf command calls into the /bin/install script in this repo, and that that's the appropriate place to add more helpful post-installation messaging? If so, I'd be happy to work up a small PR.

[jfly]

@clmay please feel free to send in a PR!

If you're just looking to use this plugig to install direnv and not anything else, I'd recommend reading #92 (comment) first.