DOC: ecosystem.rst: Koalas (#26425)

Author: HyukjinKwon

doc/source/ecosystem.rst

@@ -285,6 +285,11 @@ provides a familiar ``DataFrame`` interface for out-of-core, parallel and distri
 
 Dask-ML enables parallel and distributed machine learning using Dask alongside existing machine learning libraries like Scikit-Learn, XGBoost, and TensorFlow.
 
+`Koalas <https://koalas.readthedocs.io/en/latest/>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Koalas provides a familiar pandas DataFrame interface on top of Apache Spark. It enables users to leverage multi-cores on one machine or a cluster of machines to speed up or scale their DataFrame code.
+
 `Odo <http://odo.pydata.org>`__
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~