Added link to more real-world exercises with Flink.

L3 - Stream processing with Apache Flink/refs.md

@@ -1,3 +1,7 @@
 # Полезные ссылки по теме
 
+1. Watermarks - Practical Tool for Any Streaming and Event Processing Application by Christian Tzolov
+
 [![IMAGE_ALT](https://img.youtube.com/vi/zMgK_u_ELk8/0.jpg)](https://youtu.be/zMgK_u_ELk8)
+
+2. Дополнительные задачи для практики https://github.com/apache/flink-playgrounds.