Document how to extend integration hooks

src/content/docs/en/reference/integrations-reference.mdx

@@ -54,12 +54,18 @@ interface AstroIntegration {
         logger: AstroIntegrationLogger;
     }) => void | Promise<void>;
     'astro:build:done'?: (options: { dir: URL; routes: RouteData[]; logger: AstroIntegrationLogger; }) => void | Promise<void>;
+
+    // ... any custom hooks from integrations
   };
 }
 ```
 
 ## Hooks
 
+Astro provides hooks that integrations can implement to execute during certain parts of Astro's lifecycle. Astro hooks are defined in the `IntegrationHooks` interface, which is part of the global `Astro` namespace.
+
+The following hooks are built-in to Astro:
+
 ### `astro:config:setup`
 
 **Next hook:** [`astro:config:done`](#astroconfigdone)
@@ -613,6 +619,22 @@ A list of all generated pages. It is an object with one property.
 
 - `pathname` - the finalized path of the page.
 
+### Custom hooks
+
+Custom hooks can be added to integrations by extending the `IntegrationHooks` interface through [global augmentation](https://www.typescriptlang.org/docs/handbook/declaration-merging.html#global-augmentation).
+
+```ts
+declare global {
+  namespace Astro {
+    export interface IntegrationHook {
+      'your:hook': (params: YourHookParameters) => Promise<void>
+    }
+  }
+}
+```
+
+Astro reserves the `astro:` prefix for future built-in hooks. Please choose a different prefix when naming your custom hook.
+
 ## Allow installation with `astro add`
 
 [The `astro add` command](/en/reference/cli-reference/#astro-add) allows users to easily add integrations and adapters to their project. If you want _your_ integration to be installable with this tool, **add `astro-integration` to the `keywords` field in your `package.json`**: