alternator: expand docs/alternator.md
alternator: expand docs/alternator.md

Expand docs/alternator.md with new sections about how to run Alternator,
and a very brief introduction to its design.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20190818164628.12531-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla