docs(README): state that Sbroad is an executor for `cartridge`-based cluster,...

docs(README): state that Sbroad is an executor for `cartridge`-based cluster, because `vshard` cluster is required but not enough

README.md

 # SQL Broadcaster (sbroad)
 
-Currently the library contains a query planner and an executor for the distributed SQL in a Tarantool `vshard` cluster.
+Currently the library contains a query planner and an executor for the distributed SQL in a Tarantool `cartridge` cluster.
 
 ## Getting started