docs(readme): add note about configure_extension_registry

README.md

@@ -54,6 +54,31 @@ module.exports = function (eleventyConfig) {
 
 The `base_dir` of [convert options](https://docs.asciidoctor.org/asciidoctor.js/latest/processor/convert-options/) is relative to the document. This can be changed using above [options](#customize-with-options).
 
+##### `extension_registry` (⚠️ deprecated)
+
+From Asciidoctor.js v3.0 onwards, a new registry needs to be created and passed to the `extension_registry` option for each conversion. If you pass `extension_registry`, the extensions will be activated. Use [the `configure_extension_registry`](configure_extension_registry) function instead.
+
+##### `configure_extension_registry`
+
+The `configure_extension_registry` should be a function which accepts a `registry` (instance of `Extensions.Registry`). During each file conversion, the function will be called with a new `registry`. This `registry` instance be used to register extensions.
+
+```js
+const eleventyAsciidoc = require("eleventy-plugin-asciidoc");
+const myExtension = require("./my-extension.js");
+
+module.exports = function (eleventyConfig) {
+  eleventyConfig.addPlugin(eleventyAsciidoc, {
+    configure_extension_registry(registry) {
+      myExtension.register(registry);
+      // Or, myExtension(registry) depending on how you
+      // programmed you extension.
+    },
+  });
+};
+```
+
+Refer to [Asciidoctor.js documentation](https://docs.asciidoctor.org/asciidoctor.js/latest/extend/extensions/) to know more about extensions.
+
 ### CSS Styles
 
 The plugin does not include any CSS styles. It is up to you to style the content.