Update docs to explain usage of plugin

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Add `env` argument to [`Marpit.render()`](https://marpit-api.marp.app/marpit#render) ([#118](https://github.com/marp-team/marpit/pull/118))
 - Output HTML per slide page as array by passing `htmlAsArray` env ([#119](https://github.com/marp-team/marpit/pull/119))
 - Update docs to explain SVG slide polyfill ([#117](https://github.com/marp-team/marpit/pull/117))
+- Update docs to explain usage of plugin ([#120](https://github.com/marp-team/marpit/pull/120))
 
 ## v0.5.0 - 2018-12-28
 

docs/usage.md

@@ -199,6 +199,48 @@ The returned value is a two-dimensional array composed by comments per slide pag
 ]
 ```
 
+### Extend Marpit by plugins
+
+You can extend Marpit Markdown parser by [`marpit.use()`](https://marpit-api.marp.app/marpit#use) with [markdown-it plugin](https://www.npmjs.com/search?q=keywords:markdown-it-plugin).
+
+Due to our policy, Marpit has not extended Markdown syntax such as to break CommonMark. But you may use plugins if you want the aggressive extended syntax.
+
+For example, let's say you want to use the custom container by [markdown-it-container](https://github.com/markdown-it/markdown-it-container) to support multi-column block.
+
+```javascript
+const { Marpit } = require('@marp-team/marpit')
+const markdownItContainer = require('markdown-it-container')
+
+// Create the extended Marpit instance
+const marpit = new Marpit().use(markdownItContainer, 'columns')
+
+// Setting default theme for styling multi-column
+marpit.themeSet.default = marpit.themeSet.add(`
+/* @theme custom-container */
+section { padding: 50px; }
+.columns { column-count: 2; }
+`)
+
+// Render HTML and CSS from Markdown that includes custom container
+const { html, css } = marpit.render(`
+::: columns
+Lorem ipsum dolor sit amet consectetur, adipisicing elit. Perspiciatis
+perferendis, dolorem amet adipisci quas rem iusto excepturi ipsam aperiam quo
+expedita totam a laborum ut voluptatibus voluptate fugit voluptatem eum?
+:::
+`)
+```
+
+You're ready to use multi-column through custom container! The rendered slide is as follows.
+
+<p align="center">
+
+[<img src="/assets/plugin-custom-container.png" alt="Rendered custom container" style="box-shadow:0 5px 15px #ccc;max-height:720px;" />](/assets/plugin-custom-container.png)
+
+</p>
+
+!> Marpit has already many extends to support converting Markdown into slide deck. So some markdown-it plugins that are not created for Marpit would not work as expected because of existing extends.
+
 ## Full API documentation
 
 The documentation of Marpit API, created by JSDoc, is hosted on another site. Please refer to **[https://marpit-api.marp.app/](https://marpit-api.marp.app/)**.