docs: update README

Author: lambdalisue

README.md

@@ -5,14 +5,67 @@
 [![codecov](https://codecov.io/gh/vim-fall/fall-std/graph/badge.svg?token=FWTFEJT1X1)](https://codecov.io/gh/vim-fall/fall-std)
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 
-Standard library for using [Fall](https://github.com/vim-fall/fall), a
+A standard 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).
+[Denops](https://github.com/vim-denops/denops.vim). Users should import this
+library in Fall's configuration file (`fall/config.ts`) to use the built-in
+extensions and utility functions.
 
-It is also used to develop extensions of Fall.
+## Usage
+
+### Extensions
+
+Extensions are available in the `builtin` directory. You can access them like
+this:
+
+```typescript
+import * as builtin from "jsr:@vim-fall/std/builtin";
+
+// Display all curators
+console.log(builtin.curator);
+
+// Display all sources
+console.log(builtin.source);
+
+// Display all actions
+console.log(builtin.action);
+
+// ...
+```
+
+### Utility Functions
+
+Utility functions are defined in the root directory. You can access them like
+this:
+
+```typescript
+import * as builtin from "jsr:@vim-fall/std/builtin";
+import * as std from "jsr:@vim-fall/std";
+
+// Refine a source with refiners
+const refinedSource = std.refineSource(
+  // File source
+  builtin.source.file,
+  // Refiner to filter files based on the current working directory
+  builtin.refiner.cwd,
+  // Refiner to modify item paths to be relative from the current working directory
+  builtin.refiner.relativePath,
+  // ...
+);
+```
+
+### More Extensions
+
+For more extensions (including integrations with other Vim plugins, non-standard
+workflows, etc.), check out
+[vim-fall/fall-extra](https://github.com/vim-fall/fall-extra)
+([@vim-fall/extra](https://jsr.io/@vim-fall/extra)).
 
 ## License
 
-The code in this repository follows the MIT license, as detailed in
-[LICENSE](./LICENSE). Contributors must agree that any modifications submitted
-to this repository also adhere to the license.
+The code in this repository follows the MIT license, as detailed in the LICENSE.
+Contributors must agree that any modifications submitted to this repository also
+adhere to the license.
+
+This Markdown version will render properly when used in a Markdown environment.
+Let me know if you'd like to adjust anything further!