docs: add header image to README (#268)

Author: jackwotherspoon

README.md

@@ -1,3 +1,9 @@
+<p align="center">
+    <a href="https://cloud.google.com/alloydb/docs/connect-language-connectors#python-pg8000">
+        <img src="https://raw.githubusercontent.com/GoogleCloudPlatform/alloydb-python-connector/main/docs/images/alloydb-python-connector.png" alt="alloydb-python-connector image">
+    </a>
+</p>
+
 # AlloyDB Python Connector
 
 [![CI][ci-badge]][ci-build]