Update preview example of the action

- Use `branches` on `pull_request_target`
- Mention limitations of the preview action

See: https://progress.opensuse.org/issues/158236

README.md

@@ -54,12 +54,14 @@ jobs:
 ```yaml
 concurrency: preview-${{ github.ref }}
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
       - reopened
       - synchronize
       - closed
+    branches:
+      - "**"
 permissions:
   contents: write
   pull-requests: write
@@ -74,6 +76,12 @@ jobs:
       - uses: rossjrw/pr-preview-action@v1
 ```
 
+> [!NOTE]
+> [rossjrw/pr-preview-action](https://github.com/rossjrw/pr-preview-action)
+> doesn't support forked repositories.
+> Note the use of `pull_request_target` and `branches` as a
+> work-around with the caveat that previews only show HTML changes.
+
 ## License
 
 This project is licensed under the MIT license, see LICENSE file for details.