doc: add RisingWave to users (#775)

Author: xxchan

README.md

@@ -101,6 +101,7 @@ The Apache Iceberg community is built on the principles described in the [Apache
 
 - [Databend](https://github.com/datafuselabs/databend/): An open-source cloud data warehouse that serves as a cost-effective alternative to Snowflake.
 - [iceberg-catalog](https://github.com/hansetag/iceberg-catalog): A Rust implementation of the Iceberg REST Catalog specification.
+- [RisingWave](https://github.com/risingwavelabs/risingwave): A Postgres-compatible SQL database designed for real-time event streaming data processing, analysis, and management.
 
 ## License