docs: explain usage and note manual initialization requirement

website/content/docs/beta-features.md

@@ -664,3 +664,31 @@ CMS.registerRemarkPlugin({ settings: { bullet: '-' } });
 ```
 
 Note that `netlify-widget-markdown` currently uses `remark@10`, so you should check a plugin's compatibility first.
+
+## Custom formatters
+
+To manage content with other file formats than the built in ones, you can register a custom formatter:
+
+```js
+const JSON5 = require('json5');
+
+CMS.registerCustomFormat('json5', 'json5', {
+  fromFile: text => JSON5.parse(text),
+  toFile: value => JSON5.stringify(value, null, 2),
+});
+```
+
+Then include `format: json5` in your collection configuration. See the [Collection docs](https://www.netlifycms.org/docs/configuration-options/#collections) for more details.
+
+You can also override the in-built formatters. For example, to change the YAML serialization method from [`yaml`](https://npmjs.com/package/yaml) to [`js-yaml`](https://npmjs.com/package/js-yaml):
+
+```js
+const jsYaml = require('js-yaml');
+
+CMS.registerCustomFormat('yml', 'yml', {
+  fromFile: text => jsYaml.load(text),
+  toFile: value => jsYaml.dump(value),
+});
+```
+
+Note: custom formats must be registered before the CMS is initialized, so enable [manual initialization](#manual-initialization) when using it.