Add Spark to supported backends list (#2046)

README.md

@@ -38,5 +38,6 @@ Ibis currently provides tools for interacting with the following systems:
 - [Clickhouse](https://clickhouse.yandex)
 - [BigQuery](https://cloud.google.com/bigquery)
 - [OmniSciDB](https://www.omnisci.com) (Experimental)
+- [Spark](https://spark.apache.org) (Experimental)
 
 Learn more about using the library at http://docs.ibis-project.org.