docs: update README

README.md

@@ -7,3 +7,33 @@
 ```
 bun add --save astro-gemtext
 ```
+
+Add to `astro.config.mjs`
+
+```ts
+import gemtext from "astro-gemtext"
+
+export default defineConfig({
+    integrations: [gemtext({layout: "/src/layouts/Layout.astro"})],
+})
+```
+
+## Usage
+
+Create a `/src/pages/example.gmi` file
+
+```gmi
+# Hello World
+
+This is a gemtext file.
+
+=> gemtext.gemini README.md
+```
+
+## Configuration
+
+### `layout`
+
+The layout to use for the page. Set to an absolute import path such as
+`/src/layouts/Layout.astro`. If you do not use a layout, you will not get HMR, or the
+Astro toolbar.