Add some documentation about build modes.
Add some documentation about build modes.

This documents how to select the build mode with cooking.sh and in
what ways seastar is different from a regular cmake project.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20181219192159.15750-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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