Add sponsors footer explicitly

Since the main readme now cuts off content at the beginning of the dogfooding section.

Author: kzu

src/DependencyInjection/readme.md

@@ -1,4 +1,4 @@
 <!-- include ../../readme.md#content -->
-
+<!-- include https://github.com/devlooped/sponsors/raw/main/footer.md -->
 <!-- Exclude from auto-expansion in CI -->
 <!-- exclude -->