Readme and Setup enhancement

[AndreejS]

Hi, while setting up Scotty I encountered a couple of obstacles:

• The Readme suggests to use a pom with the version 0.3 which is not working since Scotty got updated to version 0.4
• Trying to do mvn clean install in IntelliJ would cause an error (maven version must be lower than 3.3.) if the maven version is greater than 3.3. But doing mvn clean install trough the command line would succeed, even if the maven version is not lower than 3.3. Thus Changing the version on line 145 according to my maven version solved that for me. https://github.com/TU-Berlin-DIMA/scotty-window-processor/blob/3261d0c043104941d6931f61bc6481c0804e3281/pom.xml#L143-L146
• The Flink Integration Example was a little confusing for me as a beginner since it only shows how someone with experience would adapt their code to use Scotty in their project. The FlinkSumDemo.java provided more information how to get started with Scotty while using Flink. I would suggest to add a link in the Readme which points towards the demo folder or maybe a more in depth example.