docs: add aggregations example

README.md

@@ -89,4 +89,16 @@ with SearchSession() as session:
         .fetch()
     )
     print(result)
+```
+
+## aggregations
+group by path and count unique remote_ip
+``` python
+with SearchSession() as session:
+    result = (
+        session.select(UserLog)
+        .aggregate(Terms('path').nested(Cardinality('remote_ip')))
+    )
+    print(result)
+    # result -> {'path': 1, 'path2': 2}
 ```