Add remark-github-admonitions-to-directives to list of plugins

Closes GH-1293.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

doc/plugins.md

@@ -138,6 +138,8 @@ The list of plugins:
     — add a table of contributors based on Git history, options, and more
 *   🟢 [`remark-github`](https://github.com/remarkjs/remark-github)
     — autolink references to commits, issues, pull-requests, and users
+*   🟢 [`remark-github-admonitions-to-directives`](https://github.com/incentro-dc/remark-github-admonitions-to-directives)
+    — convert GitHub's blockquote-based admonitions syntax to directives syntax
 *   🟢 [`remark-github-beta-blockquote-admonitions`](https://github.com/myl7/remark-github-beta-blockquote-admonitions)
     — [GitHub beta blockquote-based admonitions](https://github.com/github/feedback/discussions/16925)
 *   ⚠️ [`remark-grid-tables`](https://github.com/zestedesavoir/zmarkdown/tree/HEAD/packages/remark-grid-tables#readme)