doc/tutorial.md: add short intro to seastar::sharded<>
doc/tutorial.md: add short intro to seastar::sharded<>

This is a short introduction to seastar::sharded<>. It's still missing
some things - like local() (and get_local_service()), map_reduce(),
and probably other things, but we can add those incrementally.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20190821135445.865-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar