README.md: update cherries.sh instructions

Change-Id: Ibaeaf085c2347270697592c0deff4d449df68a4a
Recreate roomservice.xml before parsing it

Changes in dependencies cause duplicates to be written,
causing compile time errors.  Delete the entire file and
rewrite it before letting the script run.

Change-Id: I94c7dcb91d321df02efd29b68b5fd95d32b13b6a