add build and run from source instructions in README.md
add build and run from source instructions in README.md

fix #451

Signed-off-by: Tzach Livyatan <434a324262fc65c9776045522d30d755c828b1aa@cloudius-systems.com>

scylladb/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla