Add instruction to run in cancel_all_bookings mode to get into a clean state.

README.md

diff --git a/README.md b/README.md
index 6724a12..d35cf2e 100644
--- a/README.md
+++ b/README.md
@@ -175,6 +175,13 @@
     -output_to_terminal bool
         Output to terminal rather than a file.
 
+Plese note that this testing tool makes reservations as a test user with
+**user_id = 0**. Therefore, if you are running the tool many times, or before
+final tests, you might want cancel all previous bookings for that user to make
+sure all existing bookings for the user are deleted:
+
+    bin/bookingClient -server_addr="localhost:443" -cancel_all_bookings=true -output_dir="/tmp"
+
 Example Usage:
 
     bin/bookingClient -server_addr="localhost:443" -check_availability_test=true -output_dir="/tmp" -availability_feed="/tmp/test.json"