docs: update README

Author: lambdalisue

README.md

@@ -1,13 +1,35 @@
 # 🍂 fall-extra
 
 [![JSR](https://jsr.io/badges/@vim-fall/extra)](https://jsr.io/@vim-fall/extra)
+[![Deno](https://img.shields.io/badge/Deno%202.x-333?logo=deno&logoColor=fff)](#)
 [![Test](https://github.com/vim-fall/fall-extra/actions/workflows/test.yml/badge.svg)](https://github.com/vim-fall/fall-extra/actions/workflows/test.yml)
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 
 Extra library for using [Fall](https://github.com/vim-fall/fall), a Vim/Neovim
 Fuzzy Finder plugin powered by
 [Denops](https://github.com/vim-denops/denops.vim).
 
+## Usage
+
+Extensions are available in the root directory. You can access them like this:
+
+```typescript
+import * as extra from "jsr:@vim-fall/extra";
+
+// Display all sources
+console.log(extra.source);
+
+// Display all renderers
+console.log(extra.renderer);
+
+// Display all actions
+console.log(extra.action);
+```
+
+For builtin extensions or utility functions, check out
+[vim-fall/fall-std](https://github.com/vim-fall/fall-std)
+([`@vim-fall/std`](https://jsr.io/@vim-fall/std)).
+
 ## License
 
 The code in this repository follows the MIT license, as detailed in