docs: added extensions overview page

docs/extensions.md

@@ -0,0 +1,36 @@
+Showdown allows you to load additional functionality via extensions. You can find a list of known Showdown extensions [here][ext-wiki].
+
+You can also check the [boilerplate repo][boilerplate-repo], to create your own extension(s).
+
+## Usage
+
+=== "Server-side"
+
+    ```js
+    // Using a bundled extension
+    var showdown = require('showdown');
+    var converter = new showdown.Converter({ extensions: ['twitter'] });
+
+    // Using a custom extension
+    var mine = require('./custom-extensions/mine');
+    var converter = new showdown.Converter({ extensions: ['twitter', mine] });
+    ```
+
+=== "Client-side"
+
+    ```js
+    <script src="src/showdown.js"></script>
+    <script src="src/extensions/twitter.js"></script>
+    <script>var converter = new showdown.Converter({ extensions: ['twitter'] });</script>
+    ```
+
+=== "CLI"
+
+    In the CLI tool, use the [`-e` flag](/cli/#-e-extensions) to load an extension.
+
+    ```sh
+    showdown -e twitter -i foo.md -o bar.html
+    ```
+
+[ext-wiki]: https://github.com/showdownjs/showdown/wiki/extensions
+[boilerplate-repo]: https://github.com/showdownjs/extension-boilerplate

mkdocs.yml

@@ -39,3 +39,5 @@ nav:
     - Flavors: flavors.md
   - CLI: cli.md
   - Integrations: integrations.md
+  - Extensions:
+    - Overview: extensions.md