explain bigdata.py

p3/README.md

@@ -186,7 +186,10 @@ columns.
 individual files we upload will fit within that limit, but the total
 size of the files uploaded will exceed that limit.  That's why your
 server will have to do sums by reading the files (instead of just
-keeping all table data in memory).
+keeping all table data in memory).  If you want manually test your
+code with some bigger uploads, use the `bigdata.py` client.  Instead
+of uploading files, it randomly generateds lots of CSV-formatted data
+and directly uploads it via gRPC.
 
 ## Part 4: Locking