README cleanup

More tweaks: organized the `make` targets.
Non-blocking - I'm not sure if these make targets that actually affect a cassandra cluster (`load-dev-schema`, `migrate-dev-schema`, `teardown-dev-schema`) are by default pointed at localhost, since we're just using them in the dev vm.  They require environment variable tweaking to point at other Cassandra hosts.  Should that be stated here, since if someone tried it without the infrastructure dependencies it'd fail.