Description of plugins

### Is your feature request related to a problem? Please describe

Is it possible to add a short description for each plugin? It will be very useful

### Describe the proposed solution

Add a new column to the table (in README.md and `asdf plugin list all`)

### Describe similar `asdf` features and why they are not sufficient

There is no information about utilities at all. You have to go through two links to find out what this or that program does.

### Describe other workarounds you've considered

I click on several links.

Start with README.md and the implementation or output path for `asdf plugin list all`. Compare the plugin table and command output to determine where descriptions are sourced and displayed; done means both surfaces include a short description for each plugin without disrupting existing links or columns.

Written by the indexing model from the issue text.