start import section in README

Author: joshuadavidthomas

README.md

@@ -35,6 +35,9 @@ The library is async-only at the moment (following gidgethub), with sync support
 
    For the Webhook URL, the endpoint is up to you. See step 4 below for how the endpoint is configured. Using these installation instructions as an example, you would enter `<your project's base url>/gh/` as the Webhook URL.
 
+> [!NOTE] Have an existing GitHub App?
+> To use an existing GitHub App with django-github-app, see [Import an Existing GitHub App](#importing-an-existing-github-app).
+
 2. Install the package from PyPI:
 
     ```bash
@@ -285,6 +288,8 @@ issues = await repo.aget_issues(params={"state": "open"})
 - `owner`: Repository owner from full name
 - `repo`: Repository name from full name
 
+### Importing an Existing GitHub App
+
 ## Configuration
 
 Configuration of django-github-app is done through a `GITHUB_APP` dictionary in your Django project's `DJANGO_SETTINGS_MODULE`.