Add this plugin to `asdf-vm/asdf-plugins`

Add this plugin to the list of plugins over at [asdf-vm/asdf-plugins](https://github.com/asdf-vm/asdf-plugins). This would make it easier to install the plugin. When this plugin is listed there, the `README.md` of this project should be updated to change the installation instructions to:

```diff
## Installation

+ ```sh
+ asdf plugin add diffoci
+ ```
+
+ or

```sh
asdf plugin add diffoci https://github.com/ericcornelissen/asdf-diffoci
```

Start by checking the plugin contribution requirements in asdf-vm/asdf-plugins, then inspect this project's README.md installation section. The work is done when the plugin is listed in asdf-vm/asdf-plugins and the README includes the listed asdf plugin add diffoci command alongside the existing URL-based command.

Written by the indexing model from the issue text.