doc: Adding Feldera as known user (apache#15799)

Author: comphead

docs/source/user-guide/introduction.md

@@ -104,6 +104,7 @@ Here are some active projects using DataFusion:
 - [datafusion-dft](https://github.com/datafusion-contrib/datafusion-dft) Batteries included CLI, TUI, and server implementations for DataFusion.
 - [delta-rs](https://github.com/delta-io/delta-rs) Native Rust implementation of Delta Lake
 - [Exon](https://github.com/wheretrue/exon) Analysis toolkit for life-science applications
+- [Feldera](https://github.com/feldera/feldera) Fast query engine for incremental computation
 - [Funnel](https://funnel.io/) Data Platform powering Marketing Intelligence applications.
 - [GlareDB](https://github.com/GlareDB/glaredb) Fast SQL database for querying and analyzing distributed data.
 - [GreptimeDB](https://github.com/GreptimeTeam/greptimedb) Open Source & Cloud Native Distributed Time Series Database