Add a comparison to other projects to readme

[jspenger]

There should be a comparison to other projects in the readme (the heading currently exists, but without any body). This should be added at some point. The reason is to clarify about the differences and similarities to other projects, and making it easier to understand the core portals.

Projects to consider: Flink; Kafka; Durable Functions; feral; Kalix; Golem.