doc: Update README.md with shuffle configs (#208)

* doc: Update README.md with shuffle configs

* Update README.md

README.md

@@ -113,4 +113,21 @@ INFO src/lib.rs: Comet native library initialized
           +- CometScan parquet [a#14] Batched: true, DataFilters: [isnotnull(a#14), (a#14 > 5)], 
              Format: CometParquet, Location: InMemoryFileIndex(1 paths)[file:/tmp/test], PartitionFilters: [], 
              PushedFilters: [IsNotNull(a), GreaterThan(a,5)], ReadSchema: struct<a:int>
-```
+```
+
+### Enable Comet shuffle
+
+Comet shuffle feature is disabled by default. To enable it, please add related configs:
+
+```
+--conf spark.shuffle.manager=org.apache.spark.sql.comet.execution.shuffle.CometShuffleManager
+--conf spark.comet.exec.shuffle.enabled=true
+```
+
+Above configs enable Comet native shuffle which only supports hash partiting and single partition.
+Comet native shuffle doesn't support complext types yet.
+
+To enable columnar shuffle which supports all partitioning and basic complex types, one more config is required:
+```
+--conf spark.comet.columnar.shuffle.enabled=true
+```